There are two means of applying oxalic acid. The first and easiest is to trickle a solution of oxalic in sugar syrup that one can buy from any good bee equipment company along with the instructions and a syringe for trickling it over the seams within the hive where bees are. There is a specific dosage but there is no danger to the user other than not doing something blatantly stupid, like swigging it. The problem is that, because one should not normally do it until well into the winter (end of Dec or early Jan) when there is minimal brood, one has to open the hive up and that chills the poor little souls.
The other method that I use is sublimation - or evaporation - of crystals under the brood box which in effect fumigates the whole hive without the need to open up, thus no chilling. In my case I own a Varrox evaporator and instead of shoving the thing in through the entrance, and frying a few bees in the process, I slide a sheet of metal in the OMF varroa tray slot and place the evaporator on that.
Doomsayers will say evaporation of oxalic is dangerous. Again, that only applies if one is stupid in its use. There is oxalic in rhubarb and carrots amongst many other products one eats and they do not kill, so no worry imho. After use by either method it is advisable to give the bees a treat with some fondant, just in case so to speak.
Evaporators are very expensive but there is a Canadian version which, including shipping, works out at about 60% of the Varrox. Google surfing will find it.
Last, OXALICcan be done more than once depending on the mite drop. It also sets the colony up for the forthcoming season and is THE essential winter treatment.