Just to straighten some things out, combos are non-existent in Brawl. This is Project M, where combos are existent and bad decisions are going to you punished, and punished horribly at that. Victini is right to an extent though, and for that reason I like to think of combos as a string of perfect or optimal punishes. Thinking in that manner I refer to comboing as my "Punish Game".
Fast Fallers fall easy prey to UThrow > Uair/Fair > regrabs and aerial continuations. Comobing fast fallers you can practice because they are actual combos. To practice comboing fast falling characters you can use CPU's to an extent because there isn't much a human player can do to DI out of a competent Mewtwo's punishes anyways. Once again this is to get a feel of comboing fast fallers it won't guarantee you combos in all situations against a human player. After being exposed to enough situations, you will become more adapted to what punishes are available to you, and which ones are most abuseable in those situations.
Comboing other characters works differently but in a way the same as well. Jab > Dtilt, Jab > Grab, Jab > HC Fair and other follow-ups can be actual combos in a plethora of situations. To continue following up on characters that aren't fast fallers, you want to position yourself optimally to successfully make the next punish and repeat it until you can't. Mewtwo's insane range eases the problem of positioning, but that doesn't mean it isn't necessary to position yourself to the best of your abilities. Next you need to THINK. Think quickly and adequately, and react to the situation that the opponent's DI has created. That means thinking of their available options, your available options and which option will work best in the situation to give you the punish. That step is the most important and hardest in any form of "comboing". Sometimes this step will be a nearly brain dead punish, at other times it will require good thought process to net a punish.
This isn't only for Mewtwo it's for every character you will ever play. So to summarize, you might catch on to simple combos quickly. But to do these 0%-Death combos you see M2K and other high level players perform it will take time. Your punish game will have to develop. Not only because you have to be exposed to so many different situations. But also because there are so many different ways each situation can be reacted to. It's what makes Smash, Smash. It's also the reason competitive Smash is so good. I probably broke that down way more than I should have, but I just like to be informative in my posts. Good luck bruh.