You should do this on a step, curb, etc...(anything except rails) starting off. This is because if you learn on a rail you won't be able to do it on a step. Anyways find a step that will be easy for you to jump up on. Once you have found a good one try just stalling (landing a grind without sliding) on it first. Make your feet about a shoulder length apart, bend your knees and keep loose and relax. Jump on it with confidence. Make sure you lock on good. Once you can lock on good it's time to grind.