[StevenXenoverse]

Buy a bottle of vegetable glycerin.
Once you have it, clean yourself out well and do any prep you can to avoid a mess. Start playing with your ass and loosen yourself up/ get horny. Don't stretch yourself; just play enough to feel good and relax. Put a little glycerin on your fingers and finger yourself. You should cramp slightly, like with toothpaste but without the burn (or danger). Put some pillows under your lower back to raise your butt and let your legs fall by your head. Finger yourself some more, your butt should be relaxing and feel like it's trying to expel, but nothing should be coming out. Now, spread your hole with 2 fingers and pour in a little glycerin. Lube your inflatable plug with half lube half glycerin and insert it. Pump it until you couldn't push it out if you tried.
Congrats, you are a container. Play with the rim of your butt and even insert your fingers around the edge to circle inside or play with your prostate if you like. If you start to relax more, give it another pump. Edge 10 times, then deflate (if you want)

P.s.: Remember to put the lid back on the glycerin every time, it's hard to clean up