A slam is a fast throw made over another ball, directly into the other hand. 'Nuff said.
Begin with two balls, one in each hand. Hold one arm at waist height as you would when juggling a Cascade & hold the other up in front of you with the elbow pointing straight out in front & the hand pointing straight up. Hold the ball in this hand between the thumb, fore & middle fingers.
Throw the ball in the lower hand to the other side with a normal cascade throw. Throw to where the opposite hand would be if you were just juggling a cascade, don't throw to where the other hand actually is. It should peak at roughly shoulder height & beside the forearm of the arm that is being held up.
At that point throw the ball diagonally down over the peaking ball into the opposite hand & Claw catch the first ball all in one downward movement of the arm. Keep your wrist straight, don't bend it otherwise you will limit the amount of space you have to catch the first ball. Bring your forearm down like a windscreen wiper blade whilst moving your elbow out to the side at the same time so that the hand travels down in an almost straight line.
It is not a throw & then a catch, everything happens in one motion. Remember to throw from the fingers, time & position your swing so that as soon as the ball has left your hand, you catch the first ball in the palm.
Now do the same move but start with both arms at waist height. As you make the cascade throw carry the other ball up into position, then make the slam & bring the clawed ball back down to waist height.
Practise on both sides.
Now take all three balls & juggle Two in one hand, columns will help for when you learn the Slow Shuffle & rolling in will help when you learn The Fast Shuffle. So practise both. Hold the third ball in the other hand.
Keep juggling then throw one ball over to the other side as a cascade throw (Over the top if you are juggling the rolling in pattern). Bring the other hand up, make the slam & at the same time throw the ball in the awaiting hand straight up to make way for the slammed ball. Continue juggling two in one hand.
You can also fit a slam into a cascade. As you throw a ball, lift the next ball up to meet it, throw the slam over it & throw the next ball Over the top at the same time, then continue juggling.
When you can comfortably perform single slams on either side move on to The Shuffle.