I think it’s mainly practice (I always photograph my left hand, though, and my right hand is usually much worse); I’ve found that I can get a rounder edge along the cuticle if I drop a small dollop of polish a few mm away from the cuticle (to avoid flooding), pushing it down towards the curve, and then pulling it back up to the free edge.

I’m usually too lazy to clean up unless I touch a cuticle, but  a small brush + some pure acetone really helps shirr things up if I flood a cuticle or get a sloppy edge.

The $1 concealer brush from E.L.F is really awesome for clean-up— you can get it online at eyeslipsface.com, but they also carry it at most Targets. Any thin, stiff, short-bristled brush works, though! Just dip it in acetone and trace along the edge of the nail while the polish is still wet/tacky, and you should get a really clean edge.