There are a few purpose-made tools out there for cutting squeegee blade material. They look like oversized tin snips with a flat surface for the squeegee blade to rest. You can find these on eBay and other sites. Search for: screen printing squeegee cutter. There are a few variants out there that will do the same thing.
(Images: eBay.com)
You can also use tools similar to tin snips to cut the blade, but it is challenging to get that straight clean edge you need. In a pinch, you can use a straight edge ruler and a box knife, but this can be dangerous as the squeegee blade is quite thick and you will need multiple passes of the box knife to cut through the material. The chance for the box knife to slip is high and not recommended if you have other options.
