This is how I trimmed my biscuit.

To trim the back I took pair of small scissors that had a small tip and simple cut the bristles as near the metal ring as possible. I only cut a few bristles at a time, rotating around the ring until I had several layers cut away and the back of the biscuit was flat. This took a few minutes.

To cut notches for the vanes, I took a very sharp, small pocket knife and cut the bristles off near the metal ring, by poking the kife all the way through and using a sawing motion along the ring. This part went very fast and only took a minute or so. At first, I worried that taking out so many bristles would weaken the support for the arrow, too much. I've shot several thousand shots through the biscuit after the modifications and it supports the arrow well enough and it shows no noticable wear.