You put it straight up to the hair you want to use it on. So e.g. if you are doing the tail, hold it with the handle down in the same direction of the tail and draw it (comb it) down. Just try it on a bit that doesn't matter to much and don't worry, you won't do any harm
If you are using on a mane, you brush mane over to wrong side first and then comb down from top of neck - well that's how I understand it! Unless you also want to thin your horse's mane I'd take care!
Precisely tiffany , Brush the mane onto the the other side , and start from the top and work your way down, bringing the hair back over just to make sure your not taking too much off at a time , you really cant go wrong if you keep stopping to check . Pics before and after