The Hackamore

The true hackamore, or bosal, evolved from vaquero horsemanship traditions.  It is a plaited rawhide noseband that fits around the bridge of a horse's nose, and is attached on each side to a headstall.   A solid knot of braided rawhide at the base is wrapped with mecate reins, traditionally made with horsehair.
It is usually one of the first pieces of headgear a trainer uses to start colts, as well as used in the transition phase between the snaffle and spade bit.
You cannot achieve suppleness, balance and flexibility in the bridle with a snaffle the way you can in a hackamore.   In a snaffle, you pull directly on the horse's mouth, drawing his head to his chest and causing him to yield to the pressure at his withers rather than the poll, which places more weight on his forehand.
The hackamore, on the other hand, applies pressure to the sensitive areas on the horse's nose, cheeks, and chin.  The horse seeks relief by flexing at the poll and moving into the pull, which promotes balance and preserves a young horse's sensitive mouth.
During hackamore training, a horse learns consistency in balance and feel, making this training stage crucial in a horse's foundation.   The type of hackamore depends on the horse's face structure, but 5/8 is a standard diameter to start with.  As for length, a 10 1/2 to 11 inch fits most horses.
When using a hackamore to start a young horse, you must start teaching him to give in to pressure at a standstill.  When you can get your horse to move its face in any direction without resistance, you have achieved lightness in the hackamore.
Horsemen have their own preferences when it comes to hackamore adjustment. Some place it higher on the horse's nose.  The reason for this is that when the reins are picked up and the horse begins to break at the poll, the hackamore will go into proper position. If the noseband is adjusted too high, the hackamore is not effective, thus the horse will never feel the pressure release and will develop resistance.  If the noseband is too low, it will cut off the horse's wind and damage cartilage.
Once the hackamore is adjusted properly, stand beside the horse and apply neck rein pressure.  Starting from the ground makes the horse understand the cue easier, then graduate into the saddle as he becomes more supple.   This technique differs from the old vaqueros starting out rank horses in a hackamore.  They would supple a horse from the back of another.  To a novice, a horse's response to the hackamore could be misleading.  If the horse responds to light pressure, it does not mean it's naturally light.  Some people ride with just their fingertips, thinking they are being light.   Make your horse aware of what's being asked and you want to get a response in achieving a high degree of feel.  You should be able to move the horse's face without any restrictions.  If you are able to do this while in the saddle, you have achieved lightness.
When the time for transition comes to move into a bridle, it mostly depends on the horse.  Each horse is unique, and some learn faster than others.  There is no set time or method in making a hackamore or bridle horse.  Do take into account what is comfortable for your horse at that particular time in its training.  It might take years for a horse to progress into a bridle.  Ultimately, there are no shortcuts when training a horse into a finely tuned team member that responds lightly to your pressure.