The two classic drills are:
1) jump off the stage, QA up and then angle down to the ledge, repeat until consistent and then try it from different points below the ledge. your goal is always to be moving downward when you reach the ledge, so your first QA should send you above and farther away from the ledge, and your second should send you down into it
2) stand on the stage and try to snap to the ledge. it's easiest to do if you jump first or start on a platform, and if you're not very close to the edge, but as you build control you can try moving closer to the edge and staying grounded to give yourself a challenge. once you get the hang of it, practice this on Yoshi's Story too because the angled ledge can change your QA angle, and also practice doing it through a stationary opponent, because the freeze frames can mess you up if you hit someone while doing the QA
bonus drill:
3) practice holding onto the ledge and just aiming for different points in space and coming back to where you started. it's pretty easy to go up-down or left-right, but if you can do crazy in-between angles and still come back to the ledge without dying you'll feel really comfortable with your QA in general