Framing a mirror can be one of the most challenging jobs; however, the finished product is so rewarding that the frustration disappears from memory.
Mirror framing frustration can be from making sure the 45-degree angles are good enough so that the surfaces of the cuts touch with enough surface area to allow for the glue between them to create a strong bond. Additionally, frustration can come from cutting the glass to fit an already rabbeted frame.
Using a miter saw can be really challenging: the fence may be slightly off; the arm may not track perfectly in line; or the calibration of the turning base may be off. There are many tips out there that can help with all of these challenges. Fortunately, any single one of these challenges or mis-alignments may not pose a tremendous problem, especially when dealing with framing that is less than 2" wide. However, distance amplifies error! Thus, trying to frame with boards that are 4" or wider amplifies the error at the heel or toe of the 45-degree cut.
You can either spend time altering one of the mis-alignments mentioned above, or you can make fine correction cuts when attempting to match all four corners. Unfortunately, making correction cuts may lead to even bigger problems, especially if the frame is already rabbeted and trying to fit a specifically sized mirror.
One way around the challenges of a miter saw is not to use a miter saw. A table saw can be a great alternative saw for framing. There are great videos out there showing this technique. I use my table saw and guide for frames that are 4" or wider. With the table saw, I'm able to verify more precisely that the blade is at 90-degrees to the table and that the guide is a perfect 45 degrees. This has allowed me to create stronger bonds at the corners and reduce my time framing.
Cutting a mirror on a straight line to fit a rabbeted frame for a custom mirror is seriously satisfying. Honestly, I think it is easier and safer than many may think. There are multiple techniques used to cut a plain piece of glass. The technique I like most uses a sharpie, vegetable oil, a fence, and a rolling mirror cutter.
After measuring, mark with a sharpie and set the fence that allows the mirror cutter to cut along the desired line. A very helpful hint is to place desired cut line at the edge of a sharp-edged corner. Then, pour a line of vegetable oil along the fence and, using a finger, cover the cut area with the oil. Take the mirror cutter and roll it along the guide, cutting the mirror in one smooth, clean pass. Do not make multiple passes or try to reinforce cuts in areas; this is not necessary and will ruin the straight line. After making the cut, snap down on the glass. Bada bing bada boom - a custom mirror.
Hopefully these tips help with your framing. Again, framing a mirror is very satisfying; I suggest you try. We frame many mirrors using barn wood as the pictures show. Using the barn wood brings its own challenges, but those can be discussed later. In the meantime, may your frustrations be light and your mirrors be strong!