Insert a catheter into your ass and drain the excess shit from your rectal canal. In Brazil, we affectionately call it a handheld bidet sprayer, but it goes inside
simply put, you wash your ass very thoroughly. Its also called douching, theres like special made spray bottles you can insert easily and spray water in until it comes out clear. You can also just do it with your shower head at higher pressure. You also take a fat dump before and make sure you dont have anything left in there. Its a bit of work commonly but its just preparation. Most people need it, some very lucky ones dont, etc.