This lesson comes from a Victor Wooten video. He compares your thumb and 
fingers to a guitar pick. The proper thumb technique (known as a "thump" 
or "open hammer") is to strike the string with your thumb, going through 
it and landing on the next string. Another part of the technique is to come 
back up with your thumband sort of plucking it with your nail. The other element 
of slapping is popping, or pulling the string with your pointer finger and 
middle finger and letting it "pop".

The first exercise is to thump up and down and then pop twice using your pointer 
and middle finger. Start out slow.

TD — Thump down
TU — Thump up
P — Pop

G——————7—7———————9—9———————10—10—————————————||
D——————————————————————————————————————————O—||
A—5—5———————7—7———————8—8——————————————————O—||
E————————————————————————————————————————————||
 TD TU P P TD TU P P TD TU P P

The next step up is to do this same thing, except on the same string.

G—————————————————————9—9——9—9——————————||
D——————————7—7——7—7———————————————————O—||
A—5—5——5—5————————————————————————————O—||
E———————————————————————————————————————||
 TD TU P P TD TU P P  TD TU P P

Now, add some notes to this technique. This is the G major scale.

G—————————————————————————————————————0—0——0—0——————————————
D—————————————————————0—0——2—2——4—4———————————————————————
A——————0—0——2—2——3—3——————————————————————————————————————
E—3—3—————————————————————————————————————————————————————
 TD TU P P TD TU P P TD TU P P TD TU TD TU P P

I think this should help. I'm sorry I spaced the numbers out so much. I was 
just trying to line them up with the letters. If you have any questions or 
comments, email me at [email protected]