I would highly recommend getting a scriber of some kind. I use a scriber from Squadron mail order. It highly resembles one of those dental probes. Some folks have access to those and sharpen them up a bit. Others use a pin chucked into a pin vise.
All of these options are easier to control than the knife when it comes to scribing - it's also safer... if you slip with a scriber, you might poke yourself and it might be mighty uncomfortable, but you're not likely to slice yourself!!
Also look into something like a flexible metal ruler to help as a guide. Somewhere out there is a metal template that serves as straight edge and also has various shapes and sizes for access panels and whatnot. I saw them advertised once but forget who it was that made them.