^Agreed with Kipcom, to whom you should pretty much always listen. The good thing about tap jump is that not only does it speed up the process; it also gives you some highly beneficial directional control in the air. It can make things a bit safer on shield, shift momentum for mixups, or extend combos better. I know when I personally DJC, I have tap jump on and my input for, say, a DJC fair fading backward, facing the left side of the stage, looks like this:
->
->
(X can be used to jump as well obviously)
For mag into DJC aerial, it took me a while to find a system that worked for me. I found that using Y or X for the second jump made me go higher than I liked, for some reason. So if I wanted to do the same DJC fair backward, but out of magnet, I'd use:
->
+
->
->
This would be my using my right thumb to tap Y for an aerial magnet, then left thumb on the control stick, right thumb to press B. Then my left thumb taps upward on the control stick, and my right thumb moves downward to quickly flick the C-stick.
It's all down to taste, really. It's like how some people prefer X to jump and some people prefer Y. Grind it out for a while and find out what lets you do it the quickest, but also the most consistently. That's probably the most important thing to work on when practicing; be consistent. You don't want to leave matches down to that chance, obviously.