I don't know if I hate cutting mats because I only do it a few times a year or if I only do it a few times a year because I hate it. If I did it more often I'd certainly be better at it which theoretically would mean it was easier for me. I mostly stick with standard, easy to find sizes. But once in a while I go off my rocker and want to do some goofy size and convince myself that it'll be great to save a bunch of money doing it myself. I just finished cutting a bunch of mats tonight and probably saved about $100. But now I'm all grumpy. If you've likewise taken leave of your senses, here are some tips (to supplement the instructions that come with your mat cutter) that may help you keep your sanity:
Use sharp knives
You'll need two knives: a mat cutter, which is a specialty tool that cuts those beautiful 45 degree angles for you, and a straight knife for cutting the outside edges. There are a lot of different styles of knives (I use a simple freehand cutter and a set of Exactos) but the single most important factor, the one thing that will make the difference between sweet success and miserable failure, will be their sharpness. I'm talking crazy, dangerously sharp here. You want knives so sharp that you're actually afraid of handling them. "On the edge of death" sharp.
The knives should sail through the mat board. If you find that cutting is difficult or you aren't getting smooth edges or you're just generally having a hard time, I'd bet good money that the cause of your problem is a dull knife.
Use a large, sturdy straight edge
I use a two-foot aluminum square for my straight edge. You really want something large and strong because you'll be working with large pieces of mat board and very sharp knives. It's also a lot easier to hold in place and safer if you can use your entire hand, so the wider the better. Mine is marked in 16ths but you won't need that level of precision. 1/8" markings are fine. Using a square makes it easier to draw lines that are perpendicular to the edges of the mat without a lot of fussing around.
Use your frame backing as a template
Your mat should be the same size as the frame backing. Pop that out of the frame, line it up in a corner of your mat board and trace around the sides. Cut that out and you've got your work piece and you haven't even measured anything yet. There are a couple of approaches you can take for measuring the window but the simplest is to just put your photo onto it, measure the distance to each edge so you're sure you're centered, then trace around it. I like about a 2" border all the way around, sometimes slightly larger on the bottom edge. Your window needs to be about 1/4" smaller than the photo all the way around so now you just have to measure 1/4" marks around the inside of that first rectangle and draw the new, slightly smaller rectangle. Remember, you're making all of your marks on the back of the mat.
Use the same template for every mat
Once you've finished your first mat, you can now use that as a template for creating more. Put it in a corner of your mat board and trace the outside edge and the window. Then cut. Simple. If you're cutting a lot of mats this way, make sure you always use the same template for each one. If you keep using each newly cut mat as a template for the next one you may find that your mats are getting slightly larger with each generation.
If it's your first time, you're going to screw up. That's okay. Practice with the mat cutter on some scrap board until you can make clean straight cuts in a single pass. Once you're starting to feel like a samurai, move on to the real thing.
Use sharp knives
Have I mentioned that one already? Seriously, this is more important than anything else. If you don't have a sharp knife, don't bother. Even if you don't get so frustrated you want to take your own life, you'll never get decent results with a dull knife.