If you are starting to learn the split step. You should do a small hope in the the air and land on the ground as the opponent hits the ball. I don't think you should jump too high but if you hardly leave the air, there is no point of it.
Doing the split step is supposed to improve reaction time by alot and if you are just starting out, it will take a while for it to be natural for you. Keep doing it and don't try to be lazy.
You also want to split step on every single shot your opponent hits whether a it's groundstroke, serve, volley, etc..
Another way to sort of know when to time the split step is to watch the pros play and try to split in the same rhythm one player does when his opponent strikes the ball.
Hope this helped.