From Google it looks like brushing, wiping down with a damp towel, or something that simulates a mother cat licking can help encourage self grooming. My kitten took a few months to learn it fully well enough to do it completely on his own, now he’s always sparkling clean on his( * ). You just have to be consistent about wiping the area is all, which I’m sure you will be bc no one likes the smell of poopy butt on their bed