To do an elbow bounce while juggling, skip one throw - the ball that you would have thrown stays in your hand until two beats later. Instead of throwing the ball, use the elbow of that arm to hit the next ball that arrives on that side of the pattern, so that it bounces back up and goes back in the direction of the hand that threw it. If you're juggling more than 3 balls, the throw that goes to your elbow should be a little lower than the rest of the pattern, because it needs to get to your elbow sooner than it would get to your hand if you were just doing the basic pattern.
One way to do an elbow bounce is to bounce the ball off of the outside of your elbow, with your arm bent, your hand facing down, and your elbow sticking out to the side. Another way is to bounce it off the inside of your elbow, with your arm held out in front of you - straighten your arm, reaching forward with your hand facing up, so that the inside of your elbow comes up and hits the ball.
If you want to continually do elbow bounces, you can hit the ball straight up so you can keep bouncing it on the same elbow, or bounce it back and forth from one elbow to the other, or juggle with one or both hands, with every throw bouncing off of an elbow.