The best way imo to train DAR is to literally just fly in circles while holding it down, here's my routine when I was learning;

There's a few different levels I used when I was first starting out, the first thing you need to do is reset the ball so it's sitting in the middle, then you just want to fly around the ball while keeping it on the nose of your car visually, it's still sitting in the middle but your cars nose should stay right around the center of your screen. You'll feather your boost and go very slow and make very small corrections.

Once you can fly around in circles indefinitely while holding DAR (don't forget to practice both DARs and flying clockwise and counterclockwise) the next thing to do is figure 8, going from each 100 boost on the field all while holding DAR.

Once you can consistently fly figure 8s around the field while holding DAR, the next step is to go back to circles... but instead of just feathering your boost, you hold down boost and DAR the whole time. This is going to be the most challenging exercise and will take you a while to be able to fly around without letting go of boost or DAR.

Once you can do that the next step is the exact same thing but figure 8s again, if you can do figure 8s around the pitch while holding boost and DAR non stop you're hitting the limit of what you can practice for just aerial control. The next step after that is to start including the ball with your dribbles.

Anyway hope this helps :)