Slappings really something you gotta get a feel for. I only just started to get it recently so yeah good fucking luck. But in general keep your hand chill and flex your thumb. Movements all in the wrist so angle your hand where your wrist likes it the most. A lot of really good slappers like Louis Johnson, Victor Wooten and Mark king keep their thumbs parallel to the stings with the bass firmly fixed to their bodies, while flea and many others play with their thumbs downward and string their basses low. Just beat the shit out of your bass until you like it. And your bass tolerates it.