This trick is really easy to learn, especially if you know how to do an acid miszou (porn star). So if you don't know how to do those, learn those first before progressing to this.

The only difference between this and the porn star is that your back foot is on its laces when you do this grind. To get down on your laces, you have to squat down very low (almost like you are kneeling) and tweak that back foot over. And just like a porn star, more weight should be on the soul foot than the back foot. However when you do a sidewalk, you need to crouch down low and put a bit more of your weight on that back foot. As you crouch, you need to try and sit down on it, but try and keep centered over the rail so you don't fall. I guess the fastest and safest way to learn this is to start with the stall. The lower you can be when you land the stall, the easier the grind will be. So work up from a porn star stall to doing a full sidewalk stall. After you are comfortable with them, then you need to take the next step and do it on a rail.

Thats it!