As far as dusting, use 5% sevin powder. Put it in an old pillow case....put the chicken into the pillow case up to their neck. Hold the pillow case tight around the chicken's neck, but not too tight, and shake the chicken around to get the dust thoroughly over the entire chicken. Then, take a little of the dust and spread it around on the chicken's head. Or, you can put the sevin dust into an old sock and powder puff the chicken all over. Pay careful attention to under the wings and around the tail and vent.
Make sure you wear a mask while doing this as the dust will be flying :)
As for leg mites, the sevin dust will do nothing for this. You need to put something oily all over the legs. You need to suffocate the mites that have gotten under the scales. I used bag balm and it seemed to work ok. Then, I decided to try a different approach and used ivermectin pour on. The ivermectin pour on really did a much better job and getting the job done and along with the bag balm seemed to really take care of the problem...you need to continue the treatment until all the ugly scales slough away and new scales can be seen underneath.
Hope this helps. You can get a lot more info if you search scaly leg mites in the search bar.
I also sprinkle sevin powder all over the coop, floors, roosts...etc.
Good luck and hope this helps.