Canceling is simply doing a move before the animation for the previous move ends. So if you go to training mode and do a crouching medium kick, wait for it to end and then do a fireball, it will be two separate attacks. However, if you do a crouching medium kick and input the fireball command before the leg is retracted (while the dummy is still recoiling from the hit), it will be a 2-hit combo. That's it in a nutshell.
It's actually even easier to cancel into V-trigger. Do a normal that's cancelable (standing mp probably works for everybody in the game) and then hit HP + HK before the animation for the standing MP ends. It has to be done quickly. If you do it right, V-Trigger will activate while the opponent is still in hitstun, and this will allow you do do another attack (or even the same standing MP).