Divide clumps of snowdrops while they're still 'in the green' that is, while they still have their leaves. It's by far the best time to do it as they're still growing actively, putting down roots before they die down for the summer.
Choose the place you're going to plant them carefully: they're woodlanders, so prefer dappled shade (they're happiest planted in carpets around the feet of trees or shrubs). Clear any weeds, and dig in a good few forkfuls of well-rotted organic matter.
Then lift your snowdrop clump carefully, breaking as few roots as possible. Gently loosen the soil away and then break up the clump into little clusters of two or three bulbs. Plant each cluster about 15cm apart and water in well.
If you're short of snowdrops in your garden, now is also the time to buy them 'in the green' at your favourite garden centre. You can buy simple and beautiful native Galanthus nivalis, or try one of the more fancy cultivars: 'Flore Pleno' has pretty double flowers that look like green-speckled petticoats, 'S. Arnott' has strikingly large, teardrop-shaped flowers, Galanthus atkinsii is tall with elegant pointed petals, and Galanthus elwesii is the one to choose if you're growing in drier soil.