Drysuit neck seal

Hi

Can anyone help me, I need to cut down a neck seal to fit my neck, but
I've never done it before, can anyone give me some pointers, in
particular, how much smaller then my neck do i need it to be in order to
keep it water tight but not chock me to death. And what is the best way
to do it, lay it down on a flat surface and use a craft knife or use
scissors.

Darren