The easiest way is by division in autumn, which means digging them up and splitting them. Can also be done in spring, but too late now. That works for all but the alpine varieties, which don't get very large, so soft wood cuttings in early spring works for those. The larger ones with woody stems can be propagated by root cuttings.

I have found the best way to propagate them is towards the end of the growing season is to take cuttings and push them in to the soil where I have found that they will root very easily, I always do this and I end up with new plants that I keep in the house on a window sill until the following spring. during the winter I do not give them too much water as they don't seem to like standing in wet compost.

John, are you talking about summer bedding Pelargonium geraniums and not cranesbill, outdoor, perennial Geranium?