The Split S
The split S is really a vertical turnaround, instead of a the horizontal turn you learned as a student pilot. Employing a vertical turnaround makes it easier to hold your relative in-out flight path and it it also helps hold the proper altitude, which is why the split S is almost universally used in model aerobatics.
When you reach the far end of your flight corridor and desire to bring the plane back toward you, you perform thesplit S by pulling the model into a moderate climb, anywhere up to 45 degrees. Then, roll the plane on its back by applying full aileron, adjusting the wings to level. Once the wings are level, pull back on the elevator (up), which means you are now coming down out of an ordinary inside loop back into level flight , and back toward the field. Diagrammatically speaking, you will find the split S to be almost identical to a one half reverse Cuban eight.
The only difference is that it is not necessary to achieve a perfect 45 degree upward climb in a split S turn. In fact when flown by full scale aircraft, or in combat situations, the plane may simply stay in level flight, roll inverted, and then do a downward half loop, coming out in the desired opposite direction, but at a much lower altitude. But since we are flying so close to the ground, and since we are also interested in coming back at the same altitude we went out, we introduce the upward climb at the beginning of the maneuver, as discussed.
Once you have a feel for the split S, simply spend time practicing your straight and level flight corridor, back and forth. You will tend to fly too close to yourself. The 300 foot distance that is recommended for aerobatics is further out than you think. It will also take some practice to avoid getting your flight path angled away from the flight line. Strive to be parallel. As you burn off fuel, some down trim in the elevator may be necessary.