I've just treated a couple of birds I picked up recently, you can get prescribed treatments to kill the mite directly but i find the suffocating method works just as well.
I just wash the feet/legs in warm water to remove the dirt,
dip the feet/legs in some cooking oil, turn the bird upside down so the oil runs under the scales, dry the legs off and cover the entire scaled area in vaseline.
The idea is to prevent air getting in and the mites suffocate, the oil will drown/suffocate them and the vaseline seals everything airtight.
I also find this method will soften the scales and the damaged ones then fall off quite quickly.
I usually do this at least once a week, sometimes twice until it starts to clear up.
Don't use paraffin/petrol or other irritants like a lot of places suggest, they do work but they can't be comfortable for the bird.