It is often said that a plant is naturalised in a garden because it is planted to look as if it were occurring naturally. But a truly naturalised plant will seed itself, and over time will put itself where it wants to be. The problem with bulbs is that those which do not increase from seed will eventually become solid clumps which will look less than natural. The little wild daffodil, Narcissus pseudonarcissus, seeds itself prolifically, even in mown grass, and my colonies always have seedlings coming on among the mature flowering bulbs; whereas the summer snowflake, Leucojum aestivum, while perfectly happy in moist turf, never seeds for me, but builds up solid individual clumps.
Planting snowdrops or anything else to look natural is never easy. A useful set of guidelines might be:
Never plant a perimeter line, however informal the shape.
Never plant in evenly-sized groups.
Never plant those groups at the same density on the ground.
I once asked somebody to plant daffodils 'in a sort of whale shape; you know, nothing too formal, narrow at one end, wider at the other'. What I got the next spring was a yellow whale covered in dots. It has taken several years to 'naturalise' things again, by making holes mostly in the edge of the whale, but also in the middle, because varied density is really what makes bulbs look naturalised.
The textbook method of 'natural' planting used to be to take a handful of bulbs and to roll them out across the grass or soil, planting them exactly where they landed. It is a good theory. But suppose you are planting colchicums in late July into a patch of ornamental grass such as Hakonechloa. If the grass is to look attractive and stand up until December, you cannot sling anything there. With snowdrops, the time to split and move the bulbs is immediately after flowering, with the leaves on, so you cannot roll these out either. (Always buy them 'in the green' if you can, either potted or freshly lifted. Snowdrop specialist nurseries insist on it, for snowdrops hate to be warm and dry.)
The way to gain that casual look is to plant at varied densities. Consider where the focus should be. There you should plant the bulbs much closer together - sometimes two, sometimes a few in the same hole, but still at random rather than at equal spacings. You may feel you are putting in too many - usually a good sign. If planting under a tree, make the focus around or just forward of the trunk, as if the bulbs were seeding forwards like a shadow from the darkest place.
Then start to plant outwards at increasingly large, but still random, spacings. Leave the odd patch unplanted, and make a second, much smaller focus somewhere if you like. Towards the edge of the planting area, let your bulbs peter out, perhaps with the odd satellite one well beyond the main area. If you have bulbs to spare, put them into your main focus. It is easy - I've done it myself - to undo your work by going back over the whole area, adding in here and there until, before you know it, you have an even, all-over cover.
The double snowdrop is widely sold nowadays, and no wonder. The bulbs increase like dough left to prove. It is a cheerful, generous plant, with flowers like tiny perfumed cabbages. It does not have the poise of the single variety, whose petals hang so white and demure, but it is fun to have around. Like any double flower, it never looks so comfortable naturalised in turf as does a single. But if it is weight of colour you want, it is more than obliging.
The double form also comes to look congested in its clump long before the single, because the flowers are so much chunkier. If your idea of naturalising is to plant once and leave it at that, the double snowdrop will make you want to interfere again well before the single. If you do use it in grass or under trees, split it into very small clumps in the first place - say ones, twos and threes.