They're weird to learn, you have to go at them like your gapping for the rear wheel to hit slightly down the wall.
Go at it like you would a normal gap in every way, except aim to miss by about half a foot, then when your front wheel hits the wall or whatever else, grab your brake and throw your weight forward lifting the rear wheel before it hits.
Takes time to get it perfect but basics should be simple.
If it doesn't make sense i'll clarify with images.