If the ledge is smooth enough of a grind and has a beginning and end, try doing a ton of nosemannies and get progressively closer to the edge till you start grinding them. The trick feels almost identical to a nose manny, except there's the resistance of the grind you have to push against a little.
Doing them straight-on like a manny is a million times easier and best for learning, plus if you approach a little bit more from the inside, you'll be poised to wenning out of it. For the longest time, every time I tried backside nosegrinding a ledge in the middle, my body would turn uncontrollably and I'd have to bs revert out.
Hopefully this helps you in your nosegrind pilgramage