Pretty easy once you figure it out. Try this for starters:

1. On your left hand (assuming you play right-handed) press a fret down on the A string with your index finger, while damping the E string with your middle finger. Then press a note that is an octave from the note on the A string (two frets to the right, on the G string) with your ring finger.

2. While laying your palm on the E string, bring your thumb back from the strings about an inch or so, and quickly snap it towards the bass (with a little help from your wrist) so that the side of the knuckle strikes the A string. Then use your middle finger to "pop" the G string to play the octave note.

3. That is basically how to slap and pop an octave interval. Now try playing different patterns with your right hand and different frettings with your right hand until you get the hang of things.

If you get that much done, use a simple riff like such to practice with:

------9-------7----------9------7----8-
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-7-7----7-5---------7-7----7-5----6---
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