The Basics – Getting A Good “Slap” Sound
The first thing you’ll want to do is try it. Don’t bother thinking too much, just lift your strumming hand and let it fall on the strings around the same place you’d strum them and see what happens. No matter what part of your hand you use, the sound comes from the same place: the strings are smacking against the highest frets of your guitar. You can aim for the lower strings, the higher strings or all of them.
Next, take a look at how your hand is making contact with the strings and judge for yourself if you like the sound you’re getting. Using the side of the thumb (twisting your forearm) to hit the strings against the neck is a good, easy way to get a pleasing smack out of your guitar since the side of the thumb is a bit harder. You can also add your fingernails to the mix by bending your fingers in, and making contact with the higher strings. All of this applies whether you are using a pick or not.
After you’ve decided how you want to hit the strings, try doing it really soft. THEN REALLY LOUD! And then somewhere in between. You’ll want to adjust your volume and intensity with each song you’re playing. Also take into consideration where you’re hitting the strings – near the soundhole, closer to the bridge, over the neck – you can get pretty darn creative here and if you take the time to experiment you’ll certainly discover some really neat sounds.