Step 2: Find the "Clip" texture and click on it.

Step 3: Draw out a brush with the clip texture where you wan't it to block a player, in this screenshot I placed a clip on the right wall, from now on a player cannot jump over this wall anymore (unless he can jump over the clip, so make sure clipped walls are very high).

Step 4: Place 3 more clip brushes on the other walls, or whatever you want to clip.

Step 5: You can hide the clip in your 2D and 3D camera view by pressing "F" on your keyboard, then uncheck the box "Clip" - it will now hide every clip texture in your map.

Tutorial Written By: Jeannot