Some people use a fillet knife to cut the rubber, some use long utility blades, but there are other various methods, as n10spool mentioned. When using a blade, I like to use a little bit of soapy water as a lubricant.
Keep in mind, it can be difficult to cut the rubber without having a secondary finishing step, before applying a pocket facing. Of course, this all depends on the quality of work you are aiming to achieve. Most mechanics on here prefer to cut the rubber a little long, and sand it down to the finish dimensions.
It is possible to use a miter saw, as n10spool suggested, but you have to be extremely careful. If the saw catches the rubber just right, it'll tear the cushion, and maybe your fingers, into pieces.