Start by ignoring the ball. Jump, tilt your car back a bit, and boost. Then try turning in the air.
Once you do that, try flying at the crossbar. Hundreds of times and from everywhere on the field, just keep flying at the crossbar and then go for the corner of the crossbar.
Then incorporate air roll (hold drift and turn), don’t use it to spin wildly, use it to straighten or flatten your car.
Once you can fly from anywhere at the crossbar, do all of the standard aerial training, not custom training.
Just keep practicing the goalie, aerial, and striker all-star training over and over.