One of the first steps in matting is to decide what size and how many layers are going to be used. Mats can be single, double, or even triple with various amounts of inner layers exposed. My typical mat is a double with 1/4" of inner mat exposed. The width of the outer (nearest the glazing) varies depending on the of the image. Here's the table I start with:
Rule of Thumb for Mat size.
United Inches Suggested Mat
8" to 11".....................1"
12" to 17"...................1 ½”
18” to 24”..................1 ¾”
25” to 36”.................2”
37” to 44”.................2 ½”
45” to 56”.................3”
56” to 60”.................3 ½”
It comes from a blog post on AmericanFrame.com's blog. Color is of course important, but I'll let that discussion alone and move forward with the steps of actually cutting a mat.
When cutting mats, the sliding cutting tool needs to not touch the finished side. I did it the opposite way at first and suffered through unexplained streaks running down my first few mats.
When double or even triple matting, cut the inner mat(s) smaller than the outer (away from art work) smaller. I have been making them 1/4" smaller, but the exact amount doesn't really matter. A smaller inner mat makes it easier to align the Mats when taping them together later in the process.
Use the Mat cutter guide bar and scale to mark the back side of the mats before making the cuts for the window. You need these marks to know where to stop the cuts.
ALWAYS Use a Slip Sheet for Single Angled Cuts
A slip sheet, just a scrap piece of matting will do, needs to be between a single piece of mat and the cutter table when making angled cuts. This makes for a clean straight cut. I ruined too many mats skipping this important element.
My cutter has a little pin that can be pushed down into the mat before starting the angled plunge cut of the window. If this isn't used, the cutter will creep as it plunges, making alignment of cuts a matter of guess work, not precision.
In the picture, my left thumb is pushing down the pin in preparation for my right hand to execute the plunge cut.
Mark Across Finished Mat Into Window
Before finishing the cut of the window in the outer mat(s) mark across the "good" mat (to be used) and the window scrap. This marking is to make it easy to align the window scrap in the cut mat before taping the next inner mat to it.
Securely Attach Mat Layers
The inner mat needs to be attached to the outer mat and the outer mat's scrap window so that it will never move after cutting it. I now use a tape gun and high quality tape. I tried this with cheaper solutions and the results were, well, cheaper, and totally not acceptable.
I run a tape strip down the length of each side of the outer mat. I also run two strip on the window scrap. I usually do this parallel to the short side within two inches of the edge. It is important that no adheisive be present in the area where the inner mat will be visible, that's typically 1/4".
Keeping the tape used on the window near the edges allows me to layer cut off those edges and put the window into my reuse bin, for smaller mats. The window needs to be attached to act as a slip sheet, which is not needed on inner mat cuts.
Adjust the mat cutter to make a wider cut the inner window. The distance added controls how much of the inner mat will be visible. I typically add 1/4" to my previous width.
Proceed to cut the inner window in the same manner as the outer, except that no slip sheet is needed. After finishing your cuts the window should drop out cleanly, if it doesn't, you can repeat the cuts or use an Xacto or other small sharp knife to clean up the cuts.
Your freshly cut mat is ready for use. The amount of exposed inner mat can be varied as desired.