To measure go from middle of knee joint to the coronet band (Top of hoof). If you have trouble finding the middle of the knee bend the knee. Also, follow the contour of the leg, do not stretch the tape taught.
Everything CountryHaven said is correct....except you do NOT follow the contour of the leg. You measure "straight" down.
This is why you should never measure a "newborn" when their legs are still "curved". I usually wait until they are at least a few days old & totally "unfolded".