I pour about a cup of the 5% Sevin dust in the toe of an old stocking to make an easy applicator. Just shake it or pat it where it's needed. It really helps to have someone do this with you, but I've dusted my birds all by myself. Hold them by their feet so you're getting the dust on their skin up under their feathers.
You can also put Sevin dust in a canister with holes drilled in the lid, or in something like a parmesean cheese container. Sprinkle dust where your chickens spend a lot of time, in & around their nest boxes & under their roosts. Sprinkle it in the areas where they dust bathe.
Also, call the Miami-Dade County Extension Office & to find someone local with chicken expertise. Where are you in Miami? I know there are other chicken keepers in your county, there's always a nice showing at the county fair. There must be someone they could refer you to, a local vet who could advise you, other experienced chicken keepers. I am in Palm Beach County & have resource people here, but there must also be some in your area who could help you.
Check on the other sections of the forum for more help with mites. They can be a problem, but are treatable and shouldn't mean you must give away your feathered friends. Anyway, if they're so infested with mites no one will want to bring them home to their own flocks.
Let us know how things turn out, I wish you well!