Start in small nail target increments with a mentor. Personally, I did start the process with my Vet but gained more confidence (and the dog's trust) from watching others with similar issues at dog training classes, meetings, and dog shows.
I found that my years with horses hooves helped-seldom let the dog watch. I use a standing approach; reaching paw and gently pulling up, under and behind lets me see the "notch". Try to cut less than you feel. Each time you trim will recede the quick. Just don't try to trim all at once.