One thing I found really useful when I was learning (not sure if this is bad practice) was to to do everything as described, but once you've thrown the diabolo (if you're right handed), turn 90 degrees to your left, so you are looking at the axel when it comes down. I found that made it a lot easier to aim and ultimately catch.I no longer need to do this; once you have a textbook whip catch mastered, you can do it from almost any angle, any way you like. It makes you wonder how you ever found it so hard!
One thing I found really useful when I was learning (not sure if this is bad practice) was to to do everything as described, but once you've thrown the diabolo (if you're right handed), turn 90 degrees to your left, so you are looking at the axel when it comes down. I found that made it a lot easier to aim and ultimately catch.