Ivermectin is still an effective dewormer for many parasites, but used at incorrect doses or for the wrong parasites will not work, and will increased the likelihood of resistant parasites developing. For lice or mites, you will need to re-treat in 10-14 days. Oral treatments will only work for internal parasites and blood-sucking external parasites, not for dander mites. For mites, clean out your coop and treat with an insecticide.
There are several formulations.
The 1% (10 mg/ml) injectable is in a propylene glycol base, which is water soluble. This can be mixed in the water at the rate of 4 cc (ml) per gallon. Make it in small amounts - enough for one day at a time. Offer for 3 days.
The pour-on is a 5% oil-based solution. Place 2 drops on the back at the base of the neck (on the skin, not the feathers)
While Ivermectin is used in people as well as animals, it is considered an extralabel medication in chickens and there is no established withdrawal time. I would not hesitate to eat eggs or birds treated with Ivermectin myself, but would not give or sell them to others, as there is a risk that someone could be sensitive and have a reaction.
There is a product called a No Mite strip, McMurray hatchery carries it, that helps control mites when hung in the coop.
