I asked my vet that question a few years ago, when I first started sharing a house with a fluff-butted cat.
She said "oh, if you only knew how often people come in here with cats they have accidentally nicked with scissors!" I have concluded that this is not a place I want to save money by being a do-it-myselfer.

Actually, I have a miniature Wahl clippers. Holding her down seems to be the problem. I have used towels but always with a partner.

We use eyebrow scissors. Just had to groom the fuzzy butt last night, actually.

I think I'm going to just sit right down and refuse to budge.

I have a Furminator that I use on the dogs. It works a treat - shears through the dense undercoat, especially around their rear ends, without cutting, pulling or scratching their skin. Of course, keeping them still for long enough to groom them is another story!
You might also consider the Scaredy Cut if you have a cat that will sit still long enough to be groomed ...

I always used to have to get Bailey's butt fur trimmed either when she was at the vet or a couple of times at a local pet grooming salon. I bought a little clipper for her but was too scared to use it on her, in case I nicked her. Once her butt was shaved, though, it was so much easier to keep her clean. She had a big fat tummy and little short legs so she found it hard to clean herself properly.

I preconditioned Sookie with the clippers by turning them on and giving her a snack. I did this a few times yesterday and a couple times this morning. Then when she was half dozing on the edge of my bed, I put my bathrobe over her, a regular game. She gave me a good 30 seconds, and I did a respectable job. One more session should do it.

Our fluffy butted cat, Mingo, keeps himself pretty clean. I'd hate to have to trim him because he's a big bastard - strong like bear!