Brush all of the mane down. Grab a lock of the longest hair, comb the excess up, and yank out the lock you have in your hand. I usually wrap the lock in the comb once and then yank on the comb, you have less slipping hair that way and it saves your fingers a little. If it's hard to pull out (or if your horse is not happy) make the lock smaller. Rather then starting on one side and working your way down, I think it's best to do a little all over. That way if you have to stop in the middle your horse doesn't look like someone butchered their mane.