[1] This trick is cool and is not as hard as it looks.
You approach from the right. As a practice, it might be better to start with putting your board on the right side of the box and keep going using the right side since you will not fall onto the box.

[2] ****Once you get the idea, just go as if you ride on it with the front foot. Do not hesitate and it is more like riding on the box than putting your board on it. If you hesitate, it becomes easy to be swept off your feet.

[3] On the box, slide your board without opening your fore-shoulder.
Hand on there. If you open your fore-shoulder, the board will start spining.

[4] Release all the power used for holding when getting out. Get ready for landing.