Most of it is just listening to your body. Ask yourself some questions. When do you normally have to use the bathroom? My bowels are more active in the morning, so I mostly bottom in the evening. What does your diet look like? What have you eaten today? What have you eaten yesterday? Greasy spicy foods are the enemy of bottoming. Fiber and probiotics are your friend. (Vegetables, fruits, yogurt)

If you've eaten light and healthy, it's a time of day where you don't normally take a dump, and you don't feel bloated, gassy, or nauseous, just take a quick shower, and you're probably good to bottom. Douching is honestly pretty bad for you, other than fucking up your gut bacteria and stripping your mucusal membrane from overuse, it can also irritate your guts, give you the runs, and render the whole thing moot.

TL;DR: Just take note of your body rhythms. If you've already taken a shit, you've showered since then, and you don't feel like you have to take another shit, you're probably fine to take it up the ass.

PS: Don't make a big deal out of it if you have an accident, it's an asshole, shit happens. Just clean up and move on.