They need to be old enough for it first of all, growing pullets and cockerals have blood feathers and if you clip those, they can bleed to death pretty rapidly. So the age of your chickens would be helpful information.
As for how you do it, get someone to hold them, extend one wing by hand and clip the flight feathers, the long ones, the row on the end of the wing. Just one wing should be sufficient.
However some older birds have blood feathers here and there as well so be ready with pliers to pull out the bleeding feather and some corn starch to hold on the wound to stop the bleeding.