HOW TO BUTTSEX:
1.) USE AN ENEMA!
An enema is like a douche for your ass. You flood some tepid water in, and it flushes everything out. It's best to take a shower immediately after this, as TP has a tendency to have pieces stick to a wet butthole, and that's kinda gross. It's also recommended, though not strictly necessary, to remove the hair from around the poop chute as well.
2.) LUBE THAT SHIT UP
It is impossible to have too much lube, especially if it's your first time. When selecting a lubricant, don't improvise. Spit isn't a good idea until you're experienced in taking big ol' cocks in the back.
This is good advice for you too guys, you could tear that little piece of skin running down the far side of your dick head if it's too dry.
3.) START SLOW AND RAMP IT UP
Start with fingers, one at a time. Just slowly work them in and out until it becomes comfortable to insert another. After you've got two or three in there, you should be good to go with your dick. Apply some more lube just to be safe, and J-J-JAM IT IN!