I’ve used a rotary dog nail trimmer, it’s like a sander that spins and you can give a nice rounded filing to your chickens beaks and you don’t have to worry about cracking their beaks with the sander.Usually if I clip a beak I'll give it a little file- not the nails, just the beak.
The scissor-type cat claw trimmers work really well for this- loads of control- can take off very precise amounts with little risk of causing cracks.