Fortunately there are plenty of places to do the necessary research. The Crafts Council (44a Pentonville Road, London N1, 0171-278 7700), for instance, has a computer-based visual index of makers' work which you can access yourself. Alternatively, if you are unable to go in person, their reference desk will compile a list of jewellers' names and contact numbers for you. Another good thing about this service is that jewellers have to pass a selection process to get on the register, so ensuring that their work is of a certain standard. The only drawback is that not all jewellers apply - although the list is extensive, it is not comprehensive.
Liking a particular style of jewellery is one thing, getting what suits you is another. To avoid false starts, it is worth visiting one of the plethora of specialist contemporary jewellery galleries to see and try on a few rings. The Electrum Gallery (21 South Molton Street, London W1, 0171-629 6325) is perhaps the best known. Set up 27 years ago by Barbara Cartlidge, who has a missionary zeal for her subject, this gallery shows many of the most popular British designers such as Malcolm Betts and Brian Isley as well as a number of international names. (Bear in mind, though, that commissioning a jeweller abroad can give rise to complications.)
For a more British bias, with 120 jewellers' work on show, try the Lesley Craze Gallery (44a Clerkenwell Green, London EC1, 0171-608 0393), where they have rings by such luminaries as Wendy Ramshaw OBE - soon to have her own exhibition at the V&A - and Marlene McKibben, famous since the Sixties for her still fashionable Perspex rings. They also have plenty of newer names and prices range from pounds 15 to pounds 10,000.
Contemporary Applied Arts (2 Percy Street, London W1, 0171-436 2344) also specialise in British designers' work and are always adding new members to their list. (A selection process applies here too.) An added bonus is that the staff are jewellers themselves and so are able to offer valuable help and advice. You will find that staff in other specialist galleries are willing to organise commissions.
Also of help in tracking down jewellers are craft fairs such as the Chelsea Crafts Fair (Chelsea Old Town Hall, King's Road, London SW3, 13-25 October; for tickets call 0171-278 7700). Competition for stalls is stiff so the standard is high. There might even be some discoveries to be made. Another example is Dazzle, a craft fair which will be on in the autumn in London and Manchester (for further information call 01580-852503). An appealing aspect of these fairs is that you tend to meet the designers in person which gives you a clearer idea as to whether you could work with them on a design.
Another consideration is budget, which is best decided at the outset. The price of a ring can vary enormously depending on whether it is made of silver, platinum or gold and set with semi-precious or precious stones. Even a slight variation in size or quality of stones (particularly in the case of diamonds) might put something back in your price range that was otherwise unaffordable.
This is, of course, one of the advantages of commissioning a ring rather than buying off the shelf, as you have greater flexibility to get what you want at a price you can afford. Most jewellers will be prepared to change a stone in an existing ring of their own design or indeed remodel a family heirloom.
Once you get down to the nitty-gritty, make sure your finger is properly measured as ring sizes can vary greatly depending on whether you have a wide- or narrow-band ring. Also remember to take into account how much your fingers can vary in size from month to month and over the year. The jeweller should be able to advise you.
It only remains to settle your own ideas as to what you want. In the case of an engagement ring, it may be something personal to you and your partner. If, however, you are looking for inspiration, museums can be a very good source. Surprisingly, some ancient rings are contemporary in design and so it is well worth sparing the time to look. Beyond that, all you have to do is indulge yourself in the rewarding process of commissioning your very own "one-off" ring.