Is this the situation: you're trying to meteor smash someone when you knock them off the stage by using Dair?"
If so, once your opponent is knocked off the stage, grab on to the ledge. When you think the time is right, move the control stick away from the ledge. This will make Marth let go of the ledge and move away from the stage. Next, use your 2nd jump. As you are rising, use your Dair to try and meteor smash the opponent off the stage. Once the Dair attack is done, ^B back towards the stage. For the part where I said to use Dair as you are rising, it doesn't matter which stick you use because you can't fast fall as you are jumping upwards. Personally when I do this, I use the C-Stick to attack while I use the control stick to guide Marth's attack so I can connect it with the enemy much more easily + return to the stage much more safely, even if I miss. Hope this helped. BTW, I use the Gamecube controller to do this, but I don't think it should be any different from a classic controller when it comes to performing this.