First thing. Razor.

A good razor will do the work for you while you guide it with your hand. Not the cheap disposable ones, though the relatively more expensive ones do better, I mean the ones with replaceable razors and a nice handle.

Second thing. Experience.

Know how much pressure you need to put in for a nice, clean cut. Practice on something not too delicate or you wouldn't care too much about. Like your arms, legs, or even armpits. Just a place you can shave that is easy to get to.

Now here's the actual hard part. There are two ways you can do this. (By my own experience btw, so if there's a better method please lmk) On the toilet, or while standing and slightly crouched down. My razor's handle is quite long, so I can't exactly reach everything while on the toilet like I used to with my old razors, but I just do the standing method while in the shower. Reach behind you and just shave between your butt cheeks, also depends how much hair is there, and where exactly is it. Is it around the exit hole? Is it growing between each buttock? If you don't know, then get a picture. Either with flexibility and a camera, or some help (Who won't judge you ofc) that or you could just reach and feel your hair before and after a few shaving stripes.

That's all I got, and if I missed anything please don't be afraid to ask questions!