im not a great slapper by any regards but in my experience ive always found technique advice on slapping oftentimes doesnt relate to everyone, so my advice would be to practice your level of slap as much as possible and find the style of slap that works best for you, and once youve done that i would suggest doing exercises that make you slap all the way up and down on the neck and find songs that will help you work on your speed
this is a string skipping excercise but ive found it sounds really sick when you slap and its great for a slap exercise, although doing it quickly can be a tad difficult
G---------1-----4-------------2-3--------
D---2-3------------1-----4---------------
A-----------2-3-------------1-----4-----
E--1----4------------2-3-------------
and do that at half step intervals all the way up the neck to the 12th fret, or further if you want
hope that helped
