I'm dogshit at painting my nails and always pick at them anyways about a day after I paint them. But when I do paint them for special occasions and shit, I do as follows:
I paint one of my left-hand nails. When I inevitably colour outside the lines, I wrap a wet wipe around my right index finger/nail, and use my wet wipe-covered nail as a makeshift pinpoint eraser it to quickly neaten up any irregularities around the cuticles and sides while the varnish is wet. Once finalised, proceed to next nail.
Once the painted hand is definitely dry, do the same but vice versa.
Kind of unorthodox but it's so easy to just quickly wipe up the excess mess with precision, instead of trying to use fiddly tools or spending ages with shaky hands, hoping you can paint with no mistakes. It takes a couple extra minutes over just normally painting and hoping for the best, but they look so much nicer and neater, and means it's not a massive deal if you fuck up.