All a process statement does in VHDL is execute when a signal in your parameter list changes. You would have to come up with the design for your 32 bit subtractor and then figure out when you would want it to run. Then figure out the signal (or signals) that should trigger a process to help it execute.
It would also be helpful if you had some code so we can get a better idea of what you're trying to do. There are many ways to implement this...