What I recommend is that a few days before you are ready to use the barrel, give it a rinse, and then get as much 180F water in there as possible. You don't necessarily have to fill it (although you can) but you do want to rotate and flip it so that all surfaces get wet. This will hydrate the wood and will help reduce any contaminating yeast/bacteria.
Although I have stored my barrels with hot water, I have since learned that that may invite mold growth. So, after you're comfortable that the wood has swollen and isn't leaking, fill it with cold water. Hopefully at this point, your beer is close to being done.
When your beer is done, drain the water and fill with beer. To the very top. Add a bung with an airlock and montior it for blowoff.