I'm using the butterfly kick momentum steps to enter my backstep, but you can also just step back from a stationary position. There are two things you need to focus on, for the Reverse Aerial:
1. Spotting the ground, where you need to place your foot for the landing, the entire time.
2. Hitting the floor with your foot at about an 45 degree angle, to block your momentum forwards again.
If you don't focus on the ground, you flip into a Loser flip, which is cool too but not an Reverse Aerial. Focus on that spot and think hard about driving your foot there for the landing.
If you step back too far, you won't block a lot of momentum upwards and you probably won't have enough height. It's not as problematic if your step is a little bit short, but it will decrease the amount of flip momentum you could get. So try to hit the floor at about 45 degrees.