It's best to have someone show you, but I'll try.
Once you find the artery, roll it slightly so you can press it against the horse's sesamoid bone. The trick is to have the right amount of pressure - press too hard, you won't feel anything, press too lightly, the artery moves out from under your fingers.
IMO, a normal, resting digital pulse is really, really hard to palpate. I used to say that if I could feel it easily, it was probably a throbbing digital pulse.
For practice, try to find it on a horse that's just had a hard workout. If their heartrate is up, it's much easier to find.