The way I've always reduced density is to wet the piece, put conditioner in it and use tweezers to pluck the hair out. By wetting the hair and using conditioner this will cause the entire hair to slide out easily leaving no knots behind. By doing it this way you can see exactly where you need to reduce the density. You can remove a lot of hair this way in a short period of time because quite a few hairs slide out with each pluck because the hair is slippery from being wet and having conditioner in it.

Using tweezers will help grab and slide the hair much better than using your fingers. You do have to be careful with this method not to grab a "clump" of hair with the tweezers and start yanking the hair out because this will put stress on the base and could cause it to tear if you have a SFS system. Just wet and condition the hair and gently start sliding a few hairs out at a time.