A lot of the time, Andy is using his thumb on the back-beats, beats 2 and 4. Not always, but quite often. It can be disguised by all the other stuff going on, but for example, in the intro to Art of Motion, the thumb slaps are exactly on beats 2 and 4.
So a simple way to start would be to play straight quarter notes, and play actual notes on 1 and 3, slaps on 2 and 4. Play the bottom string with your thumb on 1. Then on 2 bring your thumb back down on the string - you could think of it as preparing to play the bass again on beat 3. But land hard, so that it makes a noise. Repeat. Once you can do that with a nice feel, it'll hopefully be easier
I wrote an article an article in he Jan 2006 Acoustic Guitar that demonstrates this. It's also online with audio, but no video:
http://www.acguitar.com/article/defa...articleid=6764