Get a long mirror, fill your crack full of shaving cream, bend over so you can see your ass in the mirror, spread one check, shave it nice, repeat on the other side. Pro tip: don’t fart while your asshole is full of shaving cream.
Practice. It’s not hard.
One leg on the bath ledge! Lots of slippery body wash. Put it in the crack and follow the groove horizontally
I have a little trimmer called the peanut that works great.
I don’t recommend shaving your ass crack, but if you have to, use a bikini razor. They’re made for shaving those weird crevices.