You can find this on multiple websites, but:
1. are the barrels wet or have they been dry a while?
-If wet, go to #2.
-If dry, you need to soak the staves first. You could add a bottle of similar booze and turn the barrel every day or two to make sure they are regularly in contact with liquid (make sure you soak the top and bottom also!). Star san also works, but it doesn't really flavor the wood.
2. Is the outside waxed (this slows down the O2 transfer and aging of your contents)?
-yes, go to #3.
-no, you may want to hit your local hardware store for a small block of paraffin wax (you can use candles, beeswax). Rub it into the wood and then torch it. I'll post a pic below.
3. Dump (Drink) the liquid and add your brew. You may want to reserve some of the original beer in the fridge for topping off. I do this with wine all the time and have done this with Barley Wine, but those are big-ass brews, so YMMV.
There is a lot more to learn about this, but this is what I do with my 5G barrels.
Have fun!
Reevesie
PS: DO NOT FLAME YOUR BARREL IF IT IS FULL OF GRAIN/DISTILLED ALCOHOL!!!!!! IT IS A BOMB IF YOU DO THAT!!!!