A sacrificial fence allows cuts to be made close to the blade without damaging the tool's stock fence. To cut a thin rabbet sacrificial fence would need to be used. The extra width of the blade cuts into the sacrificial fence.
Some fences have keyhole slots that allow a sacrificial fence to be put on or taken off easily. Others require saddles that fit snugly over their entire length.
The saddle fence is made from three pieces of wood. It has two sides that countersink into a top piece. Make sure it fits snugly over the original fence. Either cut into the saddle fence, or add another piece of wood to act as the sacrificial fence. When screwing that piece of wood into the saddle fence, ensure the screws will not be in the area where the blade will cut. Also, countersink the screws so they won't impede the cut.
For cutting taller pieces, add a jig on top of the saddle fence. Screw the jig directly into the saddle fence.
Sacrificial fences are also beneficial for miter saws. The stock fence on this miter saw does not reach to the center of the saw. When cutting a small piece of molding, the small piece will be unsupported. The miter saw has pre-drilled holes so one can drill a sacrificial fence into the saw to solve this problem.