I don't know how far exactly you have got with it but i'll go right to basics. Also as there are two types of way that people seem to slap i'll explain the way I play. This is most comfortable for me. Don't worry about which ever way is more comfortable for you because neither technique is better or worse than the other. Some would say playing one way would be more 'correct' but to be honest the correct way is just the way that you find is comfortable. (Play my way though its better

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To get started I'm going to assume that you have your thumb going across the strings by the sounds of it you do as you said your hitting multiple strings.
Firstly for accuracy you want to hit the string with the joint of your finger. Start practicising just a random pattern that goes up and down strings. Play it slowly but gradually try to build up speed slowly. Slapping is not sometime that will come quickly it is just something that requires practise so don't worry if your not getting straight away.
Peoples thumb shapes do vary so this might be an issue as to how your going to be slapping. I use the joint of my thumb as it sticks out more there. But the real trick is not to be perfectly accurate with how your slapping, just slap the string your going to and
with your left hand mute the rest of the strings. You'll find this technique will allow you to play more quickly without having to even think about accuracy.
You'll also notice over time your thumb will get fatter than your other thumb because you slap.
When slapping you need to make sure that you get your thumb to bounce of the string properly. Your thumb needs to bounce of the string after its hit it. Your thumb should only be in connection with the string for a very short time.
Also a very important thing is that you keep your hand pretty ridgid and keep your thumb and index locked in a pretty firm position. Then to execute the slap you twist your wrist to hit it quickly rather than moving your thumb itself.
Then when your thumb leaves the string use your index finger to pull the string and pop it. Remember your index finger should be rigid too.
With time you'll learn to loosen up your hand a bit but don't worry about that until you've laided down the basics.
Good luck
