I learned kind of an old fasioned way (at least the book is pretty dated, but dressage never really changes). I do a flat braid on horses with a pulled mane. Do the same starting braid as for the button braid. You should have a row of long braids, each with an 8+ inch tail of 2 strands of yarn hanging from it. I feed both yarn strands through a giant eyed needle, then bring the needle through the top of the braid from the underside. Now you shoud see a loop of braided hair, with two strands of yarn poking through the top of the loop, right at the horse's crest. Separate the two strands of yarn and wrap them once around the whole loop, then bring them back to the front of the braid a little further down and tie off, cutting away the excess yarn. It should look like a little diamond outline on top of a flat braided loop (My sis one described them as looking like cat turds. Once I ascertained that she wasn't insulting my braid job, I laughed for half an hour). Yarn is best, it doesn't break the hairs like rubber bands. For a horse with a really correct neckline (or a big show), take white tape (like electrical tape) and wrap one piece around each braid. This gives a line of white squares along the topline, and looks cool on a really correct neck. For a flawed neck or a novice horse with imperfect collection, etc, the tape can be left off, or black tape used. Oh, if the horse is grey, use black anyway.
I hope that made sense and isn't too dated, lol!