Although I do not generally think leashing chickens is the best idea, it can however, be done. First and most important though is know and understand that you CAN NOT leash a chicken by the neck. They hate it and will fight and pull against it and eventually strangle themselves. The only proper way to leash a chicken is to tie a length of twine or ribbon to the chickens leg, not to tightly to cut off circulation, but tight enough that the chicken can't slip her foot out. This something else they may struggle against for a bit, but should learn fairly quickly. Also, be advised that if you do leash them,, they will not be able to be kept together in a group like they are used to. Leashing them to close to each other will only get you a tangled mess and possible injured birds from fighting against each other.
As I said,, I do not generally think leashing is the greatest idea. Chickens generally stay fairly close to their feed source. But I also understand there may be instances where it is needed. My grandmother had a gorgeous Jersey Giant rooster once and a bunch of much smaller hens, white leghorns if I remember right. Anyway, he was much to aggresive for the smaller hens when he tried breeding them, and my grandmother absolutley loved him,, so rather than pen him constantly, or get rid of him, she leashed him in different spots in the yard each day thus solving the problem.
