Here it goes: Brace height is measured from the string to the "throat" of the grip/riser (where the skin between your thumb and pointer finger touch the riser when you are holding/drawing your bow). You can use a tape measure or purchase a bow square at any archery/sporting goods store. You simply stick a tape on the string and pull it to the throat of the riser. MOST, not all, bows should fall somewhere between 7" and 9". I shoot a Black Widow and the manufacturer recommended brace height is anywhere between 8 1/4" and 9". I prefer the lower range brace height. Don't know why, everybody differs. Also, the bow will have a "sweet spot" where noise and performance are best at a certain brace height. That is what you are trying to find!!!
To increase the brace height (make the distance from the string to the throat of the grip GREATER) you twist the string, thus shortening it, and causing the limbs to bend more. To decrease the brace height (make the distance from the string to the throat of the grip LESS) you untwist the string from the direction that it is already twisted, thus lengthening it and causing the limbs to bend less.
Brace height will always DECREASE on most strings as you break that string in because it stretches. Thus, it is a good idea to record your brace height and then shoot the bow. When you find the sweet spot on your bow, check the brace height often and make sure it does not significantly change. The following factors will drastically change brace height (leaving bow in hot car, never waxing your string, shooting in extremely hot weather, and just plain wearing a string out by shooting alot). Some strings do not stretch, but I am not a string expert and only shoot Dynaflight 97. When I shoot wood, I often tweak brace height from 8 1/4" to 8 5/8" to find the sweet spot where the arrow flys really true and the bow noise is low. TYPICALLY ON ALL BOWS THE HIGHER THE BRACE HEIGHT THE LESS NOISE THE BOW WILL MAKE AS LESS STRING IS HITTING THE LIMBS. Your bow will have a brace height that it likes. You should be able to contact the manufacturer and ask for the recommended brace height. When you find it, WRITE IT DOWN and then go enjoy your bow with confidence knowing that you are shooting at the brace height that the bow prefers to the way you shoot it. Hope this helps.