I suppose I could say I have always dreamed of working behind the cosmetics counter in a department store's beauty hall, but it would not be the truth.

The truth is, I avoid beauty halls and those counters rather like the plague, just as beauty has always avoided me rather like the plague. Should I have to cut across such a hall on my way, say, to buy a kettle de-scaler in the household department, as more befits someone of my age and general incompetence around cosmetics, I race through as fast as I can while striving to not make eye contact. Eye contact is deadly. Eye contact is the last thing you ever want to do. Make eye contact and, before you know it, you'll find yourself being subjected to what I call "the hostile make-over bid". This is when all the girls from behind a particular counter launch themselves at you, manhandle you on to a stool visible to all other customers, then cover you in all manner of cakey crap before standing back and chorusing: "Madam, you look wonderful", even though you now look like a sad old prostitute. Or one of those party clowns that frightens children. You may even look like a sad old prostitute who is now seeking work as a frightening party clown. Should you know of any vacancies, might you pass them on? (Unless the sad, old prostitute work picks up, I'm so going to have to get into clowning. I've already ordered the big shoes.)

And yet? And yet? When I'm asked to spend an afternoon behind a beauty counter, I don't say: "No". I don't say "no" for one reason and one reason only: I will make mischief. This does not speak well of me, but I confess it all the same. It's payback time for spending what amounts to the yearly household budget at these counters and then having the girl in the white coat say, as if she were the most generous soul in the entire cosmos: "I'll drop in a couple of free samples for you" (I should bloody well hope so! We're not going to eat for the next 12 months!). It's payback time for buying three products just to acquire a ghastly beach bag you'll never use, but strangely desire. It's payback time for all those false-bottomed jars, that may also be false-topped and false-sided and contain an amount of product insufficient to anti-age a flea. It's payback time for always being told a look is easy to replicate at home, even though, when you get home, you quickly discover it might be easier to replicate the Hadron Collider. I'm not sure how I'm going to make mischief exactly, but I certainly intend to tell customers that, yes, the mascara has been tested on rabbits and they really loved it. "Nine out of 10 rabbits," I shall inform them, "said it made their lashes appear both fuller and longer." I might also add: "Only four out of 10 mice said the same, but you know mice. They're never happy. Moan, moan, moan".

So it's off to Selfridges in London, now that most cutting-edge of department stores and where, in the old days when it was quite fuddy-duddy, my mother took me to buy my first bra (stay tuned; there may be more interesting titbits to come, and you'll kick yourself if you miss them). Its beauty hall houses 170 brands and is busy, busy, busy this weekday afternoon. I'll be working, it turns out, if you can call it that, behind the Trish McEvoy counter. This is a US brand sold exclusively in the UK in Selfridges and Harvey Nichols (and which doesn't test on rabbits). It is easy to think of make-up as small potatoes – a tenner on a lipstick here, a tenner on a bit of blush there – even though we know in our heart that it isn't, and even though the figures speak for themselves. Last year, women spent £481m on face make-up, £392m on eye make-up, £265m on lipstick and £126m on nail varnish and, it's been projected, this year women will spend £777m on facial skincare alone. Meanwhile one man, somewhere, bought a can of Lynx. Yippy dippy doo.

My mentor for the day is Rebecca Eaves, known as Becca. Becca is 26, has a background in visual effects and as a make-up artist, has worked for Trish McEvoy for six years and has astonishing eyelashes – "lustrous" doesn't even get near it – which she says are real. She says she uses the Lash Enhancer Nighttime Conditioning Treatment which, the website later tells me, is "a potent lash rejuvenating cocktail". I say: this is what makes me fed up. Just when you think you've got it covered, along comes a treatment for a part you've previously never even considered. It'll be potent nose rejuvenating cocktails next. She laughs good-naturedly. I ask her what the best thing about her job is: "Getting women to feel great and having fun and connecting with them through make-up". Worst part? "There will always be clients who are having a bad day, but it's not that often." What's the most someone has ever spent in one go? "Thousands. You may get a Middle Eastern woman who basically buys the whole works. But it doesn't matter what someone spends. It doesn't matter if someone spends £10 or £10,000. That £10 might be a lot of money to that person." What if you devote an hour to someone and they don't buy a thing? Do you think: Bitch! "No! Never!" I believe her, and feel rather ashamed.

The Trish McEvoy counter is, indeed, a lovely counter, the most beautiful and seductive of counters, brimming with transformative possibilities. They're pesky, those transformative possibilities and, like the beach bag, will suck you in every time. But look at this counter, just look at it, with its foundations and powders and eyelash serums and perfumes and scrubs and glosses and blushers and eye shadows with names like 'champagne' and 'golden apricot' and 'café latte' and lipsticks called 'sugar plum pink' and 'gorgeous blackberry' and 'Jolie' which, I'm guessing, is named after Angelina and her bee-stung lips. I've never had bee-stung lips myself, but I did once, literally, have a bee-stung eyelid and it wasn't such a great look. It was hideous, actually, and hurt.

Becca talks me through it all. Ms McEvoy is based in New York and is married to a famous dermatologist "so knows what she is talking about". Trish, she continues, "is absolutely amazing" and believes, above all else, "you must listen, listen, listen to the customer". She then adds: "Many of the women who come to us have had bad experiences, have had a look imposed on them that they just didn't want". Tell me about it, I say. Becca next asks how I clean my skin and the truth comes out of my mouth before I can do anything about it. "Um... wipes?" I say. She makes a face. "Not good, huh?" She says: "You must exfoliate. Your make-up will sit much better if you get rid of dead skin cells". Is this true or sales patter? This is what always worries me. She says she will give me a make-over, so I will know how to do it on someone else. (Really? They're going to let me loose?) She asks what my usual make-up routine is. This time, I do not tell her the truth, do not tell her that I wear make-up so rarely – "I work from home! I haven't brushed my hair since February!" – that, when I do, I have to spit in it to get it going again. (Interesting titbit 2: if you ever see me in make-up, you can be sure a significant part of it is gob.) Instead, I lie and say airily: "I try to keep it natural looking". I then add that I don't want to look like a sad, old prostitute or a scary clown and also, in case you hadn't noticed, I'm no Nancy Dell'Olio (I thought I was once but, thankfully, it turned out to be a dream). Becca does my face. Well, she does half my face. At the McEvoy counter, the make-up artist does one half, then you do the other, so you don't get the Hadron Collider experience at home. I won't go into all the details, but will say I learnt this:

* Always smooth over foundation with a brush rather than rub it in with your fingers, which is what I used to do, before wondering why it made no difference. (You've rubbed it all in, you big numpty!) .

* If you wiggle the mascara brush at the lash roots before sweeping upwards, it really does give the lashes oomph.

* A good eyeshadow base stops your shadow going all greasy and wrinkly; a look that has yet to be called 'oily lizard' but might as well be.

* Apply brusher/bronzer in a big 'C' starting above the eyebrow and finishing on the apple of your cheeks. (Sounds mad, but effective.)

* If you are told your skin now looks "all Jewy" take issue with your Becca – "What do you mean, 'Jewy'? How would you describe 'Jewy' skin?" – and generally make a fool of yourself before working out that she has said "dewy".

I am done, and I look hot or, if not that exactly, then substantially improved, and not too cakey. I think I am ready to go to work now and smile at a potential customer, who immediately turns tail and races into the arms of Bobbi Brown. The next races into the arms of Mac. So I just hover uselessly while repeatedly spraying myself with new McEvoy fragrance, Black Rose Oud, which is delicious. I watch the other girls go to it. They are enthusiastic, yes, but also sincere, I think. They are on their feet for eight hours a day, give or take. And they are not on commission. They get a salary, and that's that. I am humbled, deeply, but also under-employed. Eventually, Gillian is brought down from the Selfridges marketing department to be my 'customer'. This is cheating, I know, but if clients keep turning tail and running away, what can you do?

Gillian is young and a natural beauty, so it's not as if I have my work cut out, but I am good at the spiel: "Trish is married to a dermatologist and knows what she is talking about," I say. "Do you exfoliate? You should," I say. "With the blusher, do you want to go fresh or bronze today?" I do one half of her face, with Becca on stand-by to correct my mistakes, which are many. Poor Gillian. I give her eyeliner that looks like one of those rollercoaster lines on a heart monitor. And jab her in the eye with mascara. But we get there in the end, and I think I do good blusher (in a big 'C'). I then fuss around Becca as she gives half-face make-overs to Julie Nottingham, who works in a science lab, and Joanne Cocozza, a recruitment consultant. Julie and Joanne, two friends out for the day in London, don't usually stop at make-up counters either. "Too daunting," they chorus, "but we thought it would be fun today." And it is fun. Becca does listen, listen, listen, and they both look lovely by the end: all peachy and dewy. Do you like your job, I later ask her. "I love it." Do your feet ever hurt? "I'm used to it." Can I have some free samples? "No." Go on. I won't tell Trish if you don't. She says McEvoy don't do free samples. They don't even advertise. However, they do give away customised little pots and would I like a custom pot of their 'Beauty Booster' serum? You bet, I say. That's more like it. And then I'm off. Obviously, I bought a new mascara and a new eyeliner and some eyeshade base, but after that, I was off, and who knows, I might be back one day. "Treat us like a second home," says Becca, "and pop in anytime." You know, you shouldn't diss the girls behind beauty counters. They work much harder than you or I and absolutely believe in what they are doing. Plus, one last thing: I've lately been exfoliating and you know what? I think my make-up is sitting better.

(In the interests of transparency, I should add that Becca later sent me a quilted make-up bag full of McEvoy goodies. In the interests of self, I did not return them.)