The best method is just to hang your chickens upside down by their legs, and they will relax and you can give them a good powdering up! Don't forget to take out every stitch of bedding in the coop and run that you can, and then powder those places too. And powder again (as the directions say) to get those eggs that have hatched before they're old enough to reproduce.
I could never hold my chickens upside down by the legs. It sounds somewhat cruel in my humble opinion.
I pour the powder in a box, place chicken in box, close one side of top so chicken's head is out. Stick hand inside and rub dust all over chicken. Two people works better, one holds the somewhat still while other is reaching in slathering chicken with the powder.