The Shuffle is simply continuous Slam throws. So named because it looks like you are shuffling a deck of cards. Actually there are two types of shuffle, the Slow Shuffle & the Fast Shuffle. Both are pretty similar, so descriptions of each first then tips for both will follow.
In the Slow Shuffle every Second throw is a slam.
As you did when learning the slam, juggle Two in one hand columns while holding the third ball. The ball in the outside column will always stay in that column, the ball in the inside column is always thrown as a cascade throw to the other hand which slams the third ball over it. The rhythm you need to say to yourself is, "Up, slam, up, slam..."
If you have three different coloured balls, one ball is thrown up & down in one hand on the outside of the pattern whilst the other two circle each other, swapping positions each beat, being alternately slammed into & thrown out of the two in one hand juggle.
In the Fast Shuffle Every throw from one side is a slam.
Either start from a Two in one hand rolling in juggle or a Reverse Cascade. Throw one ball & slam the next ball over it, instead of throwing straight up from the hand that the slammed ball is about to enter, throw it as an Over the top throw so that you can throw another slam. In this pattern the rhythm is, "Slam, slam, slam..." One hand is always throwing while the other always slams, which makes this trick feel very much like a Shower.
Rhythm is the single most important thing in this trick, the timing needs to be very even, the length of time between each throw or slam needs to be as near to identical as possible. Both tricks are quite fast & involve a lot of arm movement in a small space, so accuracy is the second most important thing, every throw needs to peak at the same point each time. Keep your throwing arm down & your slamming arm up. The slamming arm needs to switch back & forth over the same path like a windscreen wiper, keep your elbow out of the way at the side so that it doesn't get caught up in the flight paths of any of the balls. Always watch the balls that you throw up, the slam throws can be made & caught blind.
Practise, practise, practise.