Practice baby steps.
Pick one note on your E string and slap slow quarter notes of that one note for several minutes. Don’t move on until you have this down perfect.
Then pick one note on your A string and slap that single note, slow quarter notes for several minutes. Again, don’t move on until you have this perfect.
Same thing for a single note on the D string. Again again, here until perfect.
Then pick 2 notes, one on the E and one on the A. First do four quarter notes on the E string note, then four quarter notes on the A string note. When you get that, do 2 and 2. Then alternating 1 and 1. Then switch between the A and D after you have that down tight.
Then experiment with notes on all three of those strings. Change up different patterns.
As with any practice, not moving on too soon is key. If you’re flubbing slaps and practice those flubs repeatedly, you’re essentially teaching your muscle memory to make those mistakes. Slow and deliberate until you get it right. Worry about playing songs after you can do this stuff in your sleep.
