One of the most common mistakes I see in backstroke is with the pulling motion. Most backstrokers pull with an arm that is relatively straight, as if the arm were an oar pulling through the water.
Why the One Arm Backstroke Drill Matters
TRC- Three Home Exercises to Build a Better Backstroke - The Race Club
How to Fix Three Common Backstroke Mistakes
Swimming : -TOP DRILLS FOR BACKSTROKE
All Strokes - Practice Pushoffs
Backstroke - A Swimming Guide by Francisco Quintas
Say Goodbye To A Crooked Backstroke In 3 Easy Steps
How to Do a Perfect Backstroke Start, Step by Step
Improving Your Backstroke: Pt. 2 of Stroke Improvement Series - SWSS