When using Sevin dust, I have a small dish of it next to me and I basically work small pinches of the stuff into the feathers down at skin level, all over the bird. Don't get it in the eyes, ears, or mouth. You don't want to create a big cloud of it that you and the bird might inhale when the chicken shakes its feathers, so use it sparingly. A little goes a long way, and it's very economical.
There have been several posts about using Ivomec against mites. I haven't used Ivomec for feather mites, but I did use it for scaly leg mite on one of my roosters, and it worked great. The biggest question mark with Ivomec--if it's given orally--is egg and meat withdrawal times. I think I remember someone mentioning a topical Ivermectin solution, though; that might have a shorter withdrawal time. I'm fairly sure there aren't any official studies on the subject, though, and that's where it gets tricky--no official guidelines. If I were going to use it on birds I kept for eggs, I'd be worried about how long I needed to toss the eggs out. Ditto with using it on birds for meat.
I like the Sevin dust because it's not absorbed through the skin. This means that there's no withdrawal time for meat or eggs. But it has to get on the mites in order to kill them. If mites are exposed to really low levels, i.e. a bird that didn't get dusted down at the skin levels, some mites may survive. Ivomec, on the other hand, gets into the bloodstream, and when the mite bites the chicken, it gets a faceful! That's why Ivomec is such an effective mite-killer: it turns the chicken into a mite death zone.