Sanitizing a wooden barrel is key to wood aging a beer or mead, an improperly prepped barrel will lead to soured beer or mead, so here is the process I use on my barrel.

You will need:
sodium metabisulphite or potassium metabisulphite (campden tablets)
Citric acid
Cold clean water

First step is to fill your barrel half full of clean cold water..

Next multiply the barrel's total volume in gallons by 0.4. Add this number, in ounces, of sodium metabisulphite or potassium metabisulphite to the water. For example, if you have a 5 gallon barrel, multiply it by 0.4 to get 2 Add 2oz. of sodium metabisulphite to your barrel

Next comes the citric acid, add 0.2 oz. of citric acid for every gallon of the barrel's total volume.so for your 5 gallon barrel, add 1oz of citric acid.

Jam the rubber bung into the barrel hole and slosh the barrel about to thoroughly mix the contents.

Top the barrel off with cold water and seal up the barrel.

Let the barrel sit for two days giving it a roll once in a while

Drain the barrel after the two days and rinse completely 5 or 6 times

Fill with beer to the top and get aging!

When I empty my barrel and have no beer to fill it with, I rinse the barrel with clean water a few times and pour in a couple bottles of so-so whiskey or scotch, then I turn the barrel a bit every day to keep it completely wet and properly swollen.