Easy to use. Just doubleclick the WB-icon. A window on the WB screen will open like this:

Draw your level holding down the left mousebutton or click it step by step. At the end you should be near where the second cursor blinks (the last line of the polygon that closes it will be added automatically). With the right mousebutton you can undo the lines. If you draw a line horizontal that is longer than 32 pixel it will become white and a bridge in the game. Make sure to go exactly the same way back through the bridge so that there is no land below the bridge.
Keep in mind that holes have to be "open" to the sky. That means that there must be a way frome the hole to the sky passing no line.
Levels will be selected randomly in the game. So if you want to test your new level, it is a good idea to move all other levels temporarly to another location on the disk.

Have fun !