Do not even consider using the Hypoaspis mites if you have used any kind of anti mite killing chemicals in the last 3 months.
It is important that the mites are provided with a reasonably humid environment but not soaking. Even desert species experience wet periods so a raised humidity for a couple of days shouldn't hurt them.
The mites don't climb very high so the snakes need to be on the ground (remove climbing branches in arboreal set ups).
The mites do their job in about 48 hours but will hang around longer clearing up any missed snake mites.
The Hypoaspis are very good at clearing up mite problems in invertebrate cages.
You may see spots of blood on your snake after adding hypoaspis (especially on white snakes). Don't panic, this is a result of the hypoaspis biting into the snake mites that were feeding on your snake and shows they are doing their job.