And if you don't know how to slap-tongue ...
I recommend starting with just a reed. Lay it flat on your tongue and flex the tongue to "cup" the reed in a suction grip. Now lean over and see if the reed falls out of your mouth. Once you get the hang of keeping it on your tongue, stick it on the mouthpiece.
Form your embouchure as usual. Apply the same suction technique, pull on the reed, and release. The sound of slap-tongue (fundamentally) is the reed bouncing back to hit the mouthpiece. Some think it is easier to add a short burst of air to the technique at first to make getting the "pop" easier. One day, it'll hit you like a ton of bricks and you'll be able to do it forever. Good luck!