Hi Mark,
I good place to start is playing some scales with just your thumb/slap as this will help develop your accuracy. Do you know if you are striking the string with your thumb parallel (Marcus Miller and others ) or across the string (Flea)? Both are fine though achieve different results. Also, have you written the bass line you want or is the aim that the slap technique will give you new ideas for sound and rhythms?
Cheers,
Simon