There's no strange science to cutting a mat for your artwork. Even for those of us who are mathematically challenged, I raise my hand here, it's really not complicated in that regard. If my Artist Attention Deficit Disorder brain can handle it, so can yours.
Here you can see I've gathered my tools: T-square, triangle, straight-edge ruler, masking tape, pencil, kneaded eraser, and the dreaded one edged razor blade. Ta Da!
Usually I try to cut a mat to fit into a standard frame size. It just makes it easier for me, or a buyer, to find a frame. I also want the mat to have 1 1/2" to 2" of border around the work. If the art is a weird size I might make the lower edged border slightly larger than the top edge.
Again, this is just pleasing to the eye. I measure my art, decide where I want the mat placed and look for the border size I want. Easy. Is there a lot of math involved? Nope.
It's hard to see in this photo, but I measured the area I planned to cut out of the board and penciled in guide lines. Then I squared up with my straight edged tools and ready to cut the bevel.
Holding the razor at an angle, I begin at the left hand corner of the border and rest against the T-square. With a new blade you can cut right across in one try, keeping the angle for the bevel, and stop right at the right corner at the next pencil line.
Control is important because the mat looks bad if there are cross cuts. And you don't want to keep going over the line you just cut because doing that can get nasty. No one wants to see scraggly strands of paper, or unevenly cut lines on a mat. Fresh, sharp blades always.
Having cut three sides of the opening, you can see a little bit of the bevel. Those lines visible at the corners are just pencil. I'll erase them when I'm finished cutting. Just to backtrack for a second, I do turn the mat around and square up to cut each side.
And Voila! The mat is cut. How nice is that bevel, huh? Nice, right? Yeah.
After I clean up the tiny bits of paper in the corners with a careful cut of the razor, I erase my pencil marks. I position my artwork where I want it to show, fix it with a bit of masking tape, then add neat tape to it. I snip off the tape with my razor leaving a clean edge all around.
Here I'm erasing my pencil lines.
Can you see the bevel on the lower edge and some of the corner? Nice, right? Boom!
The bevel catches the light, if done right, and makes the work look finished. Framed it would look even better.
And there you have it. No secrets. No crazy calculations. Just some simple numbers, a steady hand, and a sharp razor. Get some nice, white mat board, flat or with a little texture, and try it. Not hard at all.
When you find out what they charge to do this for you, believe me, you'll figure out how to do it yourself quick.