to acid grind, its basically a step by step process that will always end up with u on the ground at sum point.

 1) start by jumping onto the rail stationary nd putting your soul foot (dnt know wats ur soul foot? message me) onto the rail, box etc. as you would when you go for a sol.

2) with your other foot, place it in front of your soul foot while keeping your body upright. you may find this uncomfortable but it's the way i learnt to do it. once it's placed, put all your weight down on your rear foot and stay there.

3) once your comfortable, start doing this moving, and see how you get on.

 Hope i've helped!!!