Do you have any experience with an e-collar?
In any event, getting a sit to flush is all about obedience. Start with the pup at your side like in AlaskaHunter's video. If he won't sit while at your side, he's not going to sit 15 yards in front of you with a wild bird flushing off his nose.
Do the walking heel/sit obedience stuff and get that solid first while on lead. Then eventually, train a walk up. Basically walk with your dog at heel, then have a helper say 15 yards in front of you toss a bumper/bird in the air. Your dog should immediately sit at the sight of the toss. Once that is solid, have your helper hidden so all your dog sees is something flying out of the bushes, again the dog should sit! Add in a shotgun/pistol shot as well once things are going good. To help the dog see the toss coming out from the bushes, you can have your helper yell "hey,hey" or "There goes one" or something to that effect. Next step would be to have your dog quartering a field, then toss a bumper/bird in the air, yell "Rooster" and fire a shot. If he's steady while quartering, your well on your way to a sit-to-flush dog.
The trick is how you handle it when the dog doesn't sit. An e-collar makes that a little easier when done properly, especially for the times when the dog isn't at your side but quartering in front of you. The important part is to not let your dog get the retrieve if he doesn't sit. If your not e-collar savvy, a long check cord may be your best option.