Cutting Fabric

As the title implies, I need help cutting fabric.

I can cut fabric if I have a guide to follow. The ladies at Hawkland Moor helped me make my first tunic. They marked the area for me to follow, and I cut along the blue line. I can sew it together fairly easily, though I can be an entire seam's-width off at times. It doesn't really hurt for the level I'm working at.
If I'm working on a gridded fabric, I can cut it mostly square and put the extra in seams and hems.

Right now, I'm trying to make some gores for a dress. I can't for the life of me figure out how to cut at a diagonal across this gridded fabric. My dad says to stretch it out over a clean wooden board, pin it down, and cut it rotary-style. Not only do I not know where to find a rotary cutter, I'd have no idea how to handle it. We have lots of shears, one special, al-metal pair that my mom uses to cut mesh into strips for crocheting scrubbies, and a few rarely-used nice, sharp, long pairs. It cuts fabric well, so long as I know where to cut at. But I don't know how to know where to cut at.

So that's where I'm at right now. A few of the ladies offered to help me with it at Apple Day this Sunday. But we never get through every additional task at the A&S days. So just in case we don't get to it, I'd appreciate some help through here.

~Dartania Thorne