Dragon flags are given in the dragondoor by "Bulletproof Abs," if you want a full description.
Having read the book and done the exercise, here's my description.
Lay flat on your back on a bench and anchor your hands underneath the bench by your head. Hold on hard or you won't be able to do them. Now, concentrate on pulling your fee up toward the ceiling while keeping your whole body rigid - the only part of you that can stay on the bench is the are from your head to the top of your traps. I'd recommend starting from the top - with your body aiming toward the ceiling - and doing negatives for a while. This is an extremely difficult exercise!
Also, there is a sample clip of Pavel (author of Bulletproof Abs) doing a few exercises, including the Dragon Flag, at:
If you have questions, just ask.