Its quite simple. Please. Theres no need for specialist cutters or hammers or any old clap-trap.
Lay the rubber out flat. Make sure you have a new sharp blade in your scraper. Press the blade on the rubber at the point you want to cut it. Press down firmly.
Once you have done this you will find the rubber has been cut; cleanly, straight, with no added expense, with minimal time, minimal fuss and no need for any extra tools, hammers, cutters or other paraphanalia.
It really couldnt be simpler.