Midwinter is the best time to give the bees a final treatment for varroa mites for the year. We use an oxalic acid based treatment which will kill any mites living on the bees. I won't affect mites living on young bees inside brood cells — hence it needs to be done in a period when there is no brood in the nest.
This year we tried sublimation for the first time. This involves using a vaporiser connected to a 12V battery. The method is not without problems. The entrance has to be blocked, the acid makes the bees seriously unhappy and they try to get out.
My only tip is to choose as cold a day as possible — this is when the bees are least active and will cause the least amount of trouble.