Clipping a hens wings is often done to prevent them from flying off, but first you should make sure that you actually need to do this - a lot of heavier breeds find it very hard to get airborn in the first place!
If you do need to clip a chicken's wings, you should use a pair of sharp scissors to clip about half to 2/3 off the length of the Primary Flight feathers. These are the last 10 feathers on the chickens wing. CLIP ONLY ONE WING. This throws the bird off balance and prevents flight. If you clip both wings, the bird may still be able to fly by simply flapping harder. This is a temporary solution, because the feathers will grow back at the next molt, which may be a few months for young birds or up to one year for older ones.
A potential problem is that clipping these feathers may make them harder for the bird to shed during a molt, requiring your help.