The bs boardslide is a basic trick and probably one of the first slides you're going to learn. Since you land on the obstacle with the center of the board, there is a lot of room for sliding. It's recommended to learn the bs boardslide on a flatrail instead of a ledge because the wheels cannot get blocked there. You should be able to do ollies and fs 180s before learning the boardslide.
BS BOARDSLIDE - STANCE: REGULAR
Approach the ledge/rail at a more or less parallel angle with your feet in ollie position and crouch down.
Do a 90° ollie in the fs direction by pushing down the tail, pulling the front foot towards the nose, and turning your shoulders in order to initiate the rotation. Land on the obstacle with the center of the board.
Keep your balance such that you don't slip away or get stuck – your arms will help you out. Ideally, you slide until the end of the obstacle.
Now you simply turn your body and your board back into the starting position – parallel to the ledge. Try to land on the bolts and keep rolling. The bs boardslide is finished!