2. brill bit
3. hacksaw blade or rotary tool circular saw
4. metal file or sandpaper
5. nose plier
The bristles at the four corners that the circle intersects need to be removed completely using the nose plier.
Step 2: Making the cut
Remember to try to keep the same number of bristles / group of bristles such that it is symmetric around the center of the hole.
Step 3: The final steps
Cut off the remaining part of the toothbrush and give hand drill a few turns while holding a pencil against the remaining toothbrush. This should now mark out a circle which we need to file off.
The corners can be cut off using the hacksaw blade and a file used to give a rough round shape. Finally attach the brush to the rotary tool and spin at lowest speed setting. Hold the file against the brush to give it it's final circular shape.