Snowdrops are obliging plants, growing in acid or alkaline soil and not too fussy about position, although neither deep shade nor baking sun is recommended. Before planting, dig in some sieved leaf mould or garden compost and a handful of blood-fish-and-bone or other organic fertiliser. If you have heavy soil add some grit or sand to help with drainage. In very poor soil you could add a little well-rotted manure.
Traditionally, the time to plant or divide snowdrops is when they are "in the green": that is, after they have flowered but before the leaves have died down. You should divide clumps every two or three years, or when they become congested. Dig them up carefully, paying extra heed to the roots, as they will not regrow if broken. For a naturalistic look replant in groups of three or five bulbs, about 10 centimetres deep and with about 20 centimetres between groups, or plant in "rivers" through a border, with five to 10 centimetres between each bulb.
Put in a label when you plant snowdrops, as after the leaves die down there will be no sign that the bulbs are underground. Rather than shoving the label straight into the ground (where it will eventually be trodden on), make a skewer (45 centimetres long) from half a wire coat hanger, hang the label from it and stick it in the soil.