For such I would suggest everclear or another high-proof liquor, a funnel, and a tube that isn't too thin (may have difficulty passing through) or too thick (will cause unnecessary burn). Then lay on your side and insert the tube and attach the funnel (with tape if not snug) and pour the alcohol into the funnel.
I am actually thankful that you brought this up because I hadn't thought of it in years. I tried this many years ago but used wine so the effect wasn't pronounced enough to CTB but I certainly got quite intoxicated.
Strong liquor would certainly be the way to go but it WILL burn, there are over the counter products you can buy to numb your anal cavity though which would likely help a lot with the pain and possibly even nullify it.
