A hackamore is a piece of horseback riding equipment that can be used in place of a bit on the bridle. The hackamore goes around the horse's nose and is used to control the horse without ever placing a bit in its mouth. There are two types of hackamores: the traditional rope/soft hackamore and the mechanical hackamore, which works similarly to a curb bit. You can use a traditional western headstall with both types of hackamore.
Remove the nose band from the headstall if it has one. The hackamore goes where the nose band does once the bridle is on the horse's head, so the nose band will get in the way and interfere with the function of the hackamore if left on.
Open the snaps or clips on the bottom of the cheek pieces of the headstall, just as you would if you were attaching a bit to the headstall. Some headstalls may require the use of a small screwdriver to open them.
Fasten the hackamore into the cheek pieces. On a traditional hackamore, you will place the top/nose band of the hackamore in the end of the cheek piece and fasten the cheek piece around it as if it were a bit. Some hackamores have knots or openings where the cheek piece will fit. On a mechanical hackamore, you run the cheek pieces through the opening at the top of the metal shank, just as you would with a regular bit.
Attach reins if necessary. The traditional hackamore will have its reins connected to it already. Reins can be attached to the loop or opening on the bottom of the shank on the mechanical hackamore.