I agree with GotaDun.
If your horse is anatomically built to hold his head/neck naturally around level, when the horse learns to use his body right his head will automatically go there.
If you have to seesaw in their mouth or use gimmicks like martingales etc, your horse is probably not physically ready to put his head/neck at level.
A horse must be in really good shape and on a regular exercise/training program in order to be strong enough to go slow and put their head down. If the horse is not in great physical shape it is really VERY unfair to ask them to carry themselves correctly and go slow and level.
Teaching a horse to go slow and level generally involves A LOT of lateral work and suppling to get there.
Try to find some videos by reputable WP trainers to rent (there is a company now that you can rent horse videos, like netflix for horse people....just google for it!).
Good luck in your ventures and always seek a reputable trainer (look at their show record and level of showing, don't just take their word on it!! There are a LOT of yahoos out there calling themselves trainers!) to help you out even if it is just for a lesson once a month! If a trainer always has a horse all balled up, is continually yanking at their face, or the horse just doesnt look happy....find someone else for help because that is NOT the appropriate way to train a horse!
Look up Steve Heckaman and Dana Hokana to get an idea of what correct training looks like.
Have fun and remember a good broke horse will take at least 3 months to start getting the hang of WP and sometimes up to a year, so don't rush it!! Do it right!