I've focused by eye for 20 years. The cheap grain focusers are annoying and I'd rather do it by eye. A Nice grain focuser is indeed nice, but expensive. I got a Peak brand one from someone unloading darkroom stuff on clist. It's the one to aspire to if you want an actual good grain focuser. I don't get sharper photos with it since I can focus fine without it. It just makes things a little quicker.
Focusing by eye, you just rack the focus through the focus zone and iteratively back and forth like you would with an image on a ground glass. You watch what's happening and settle on a spot and it's good.