Firstly don’t worry too much about going over the edge if you’re just learning, it happens. Both men and women who have masturbated a lot but not practised orgasm denial are often pretty bad at holding back and keeping right on the edge. Your body has had years of learning how to orgasm, not to stop.
Having said that, the real key is normally to just slow everything down, and take your time. You need to learn to recognise the signs that you’re getting near and not just the rush of stimulation that says you’re about to cum. See if you can take 20-30 minutes building up to an edge and be really aware of how your’e feeling.
Think about putting extra layers between your fingers/vibe too, even a pair of panties can make a difference.
But really the key is practice.. just try, lots. You will get better at it!