Measuring guide for each part
A: Half noseband
Measure from one cheekbone to the other. Take the measurement approx. 1 cm below the cheekbone.
B: Chin strap for half noseband
Measure from the position of one noseband ring, under the chin, to the position of the other noseband ring.
C: Cheek piece for half noseband
Measure from the noseband ring, along the cheekbone, to the outer corner of the eye.
D: Cheek piece for headpiece
Measure from the bit ring to the middle of the eye. Note: Consider the size of the bit rings if you use multiple bits. Measuring with a bridle on the horse may give greater accuracy.
E: Headpiece
Measure from the lower edge of one ear, over the poll, to the lower edge of the other ear.
F: Browband
Measure from the lower edge of one ear, across the forehead, to the lower edge of the other ear.
G: Middle throatlatch
Measure from the middle of one cheekbone, under the throat, to the middle of the other cheekbone.
H: Tupper throatlatch
Measure from the end point of C (at the outer corner of the eye) on one side, under the throat, to the end point of C on the other side.
You can watch a full video guide here