Here I have loaded my dremel with a cutting bit and Im cutting the channels I marked with a pencil free handed. You have to be careful not to let it get out of control and go too deep or worse penetrate the sides themselves. you only want to remove enough kerfing to let the back plate rest flush with the bottom edge of the sides.

Hearing protection is a good idea as is knowing that you have a steady hand in the face of fast moving motorized tools before touching the instrument. Doing this freehanded is a bit radical, keep in mind I have been at this a while and had to cut entire fret slots freehanded with a dremel on a regular basis at my old work so I have had lots of practice. If its your first time doing stuff like this you may consider getting something like this: http://www.stewmac.com/shopby/product/5260