Answer:
First off you need to understand that this is a fetish practiced by adults who want to explore the parameters of the fetish. Using the bathroom is part of the process of exploring the fetish. Urination is usually handled by a few drainage holes in the belt. When you urinate you have no control over the process and can feel the warm urine building up inside the belt. As the pressure increases the urine flows faster through the holes in the belt. When you stop you must remain seated and allow the belt to finish up draining the urine. You can never dry yourself completely and will dribble after you stand up. This is part of wearing the belt and you accept this. To defecate through a chastity belt designed to allow this is a messy task at best. The hole in the belt cannot be large enough to allow feces to freely exit your body because something could enter her body through the same hole. The hole is usually smaller than a penis and some have teeth to prevent anal sex from happening while wearing the belt. Depending on your level of strictness, it can vary from only wiping off what you can and returning to your previous activity's to getting in the shower with the belt on and cleaning yourself the best you can or removing it ahead of time. The latter is sometimes avoided at all costs as it defeats the purpose of wearing the belt. Hygiene is important for females for obvious reasons and wearing the belt should be avoided if she is on her period. In extended sessions of wearing the belt it should be removed once a day and cleaned along with the wearer cleaning herself also before putting it back on. I'm sure there are some variations of the descriptions above but basically that's about it.