First thing you do is give the snake a good bath in luke warm water. Place the snake in plastic tub or bucket with a lid and leave him in there for a while. The water should be deep anough to cover the snakes body but not so deep that he has to constantly swim to get air.
Next, clean the cage. Remove any cage furniture and wash thoroughly. If you are using a glass cage or molded plastic cage you are in good shape. If its wood its going to be harder to get rid of them but not impossible.
For as long as your treating for mites use newspaper or paper towels as a substrate. No loose bedding as it gives the mites more places to hide. Also keep cage furniture to just necessities. Just a water bowl and a hide, thats it.
I havent treated for mites in over ten years so Im afraid I cant suggest a good product since I dont know whats out there.Back then I used something which has since proven to be harmful so I cant suggest that. Do a little research and Im sure you'll find something.
The main thing is clean, clean, clean!