Like many great exercises, performing a proper dragon flag takes practice. You might even need to do some remedial work before you're ready for it. Begin by working on straight leg raises while lying on your back. Go slowly and don't swing your legs or allow your lower back to arch.
When you reach the point where you can do multiple reps without losing form, you're ready to work on the dragon flag. Start by practicing the negative (lowering) phase of the dragon flag first. Kick up into the vertical position, and then try to lower your body down as slowly as possible.
Once you get confident with negatives, try doing a static hold at the bottom with your body hovering an inch or two over the bench. When you can hold this position for 2-3 seconds, you're ready to start working on full dragon flags.
The progression should look something like this:
While some will be ready to jump in at week 5 or 6, others will need to stick with each phase for longer than two weeks. Progress doesn't occur at the same rate for everyone, but if you're patient and persistent, your day will come.