To do a Wall Tap you...
1. Approach the wall at a fast pace.
2. When at a close distance pull up as in a manual.
3. Try to land with both wheels on the wall and you should be completely vertical
4. When trying to ride out don't try to bunny hop off, instead push off and ride off in a fakie.
5. Feel cool.
Honestly this trick can take you a long time to finally get, but don't let that intimidate you as the wall tap sticks with you and is worth all the effort.
