Bass slap

[Indianss]

Hello im starting practicing bass slap, i looked at a lot of videos on youtube, and i figured out i prefer the thumb up position, my question is, do you aim a little higher than the string you want to slap? because if i aim at the string i want to slap im slapping the string on top (example : if i want to slap A string most of the time i hit the E string aswell), i guess is just practice but some advice would help, thanks

[Shroud]

I think there's many ultra-fine details to adjust when using any slapping techniques, and these will vary a lot among individuals... just because we have different fingers, hands, wrists, arms and so on.

I am a bass beginner myself so I shouldn't shell out much specific bass advice, and in addition the "thumb up" technique seems to be the only one that doesn't work at all for me yet  I can produce a decent slapping sound using "thumb straight", "thumb down" and "hit through" techniques, but when I try "thumb up" my wrist inevitably ends up muting/muffling all the strings unless I raise my arm a bit, but then I am not relaxed anymore.

In more general terms, coming from decades of practicing guitar (but not bass), I can only suggest you to take some time off, like an hour or so, to investigate the technique in the most mindful and analytical possible way. No exercises, no metronome... just plug into an amplifier and try every small variation of your posture and movements that comes to mind, in order to seek the sound result you aiming for. If that requires you to aim a little higher, so be it! 

Remember to test it around with various notes across the whole neck, as the strings respond differently when you are pressing a high fret rather than a low fret or slapping an empty string, and of course try the techique on all the strings.