The tool is a tri-edge knife and looks primative, but it's very effective for opening the ports in this holder. 

The tool is about not cutting with the blades edge. The entire process is all about taking your time for best results. 

The ports in the insert have a taper to them  \_____/. The process is about mainting this same taper while opening the port. 

Place the bit so that a flat side of the blade rest along the inside edge of the hole matching the angel of the taper. 

Rotate the blade with outward and a slight downward pressure into the plastic to create a scraping action, work this a full 360 degrees to keep the hole round. 

After each pass check the port for fit and repeat as needed.