I made mine at TechShop.
Sheet of mirrored acrylic (not a glass mirror!)
For those wondering about using real mirrors, I actually tested the rear-etching process on a chunk of real mirror, but the etching quality was quite poor. Also, each magnet would have to be cut by hand. I don't have a glass grinder, either, so the corners would always be sharp, which is not as nice.
Step 1: Design the Artwork
In a graphics program that can produce vector files, create a red rectangle with the determined height. I like to round the corners slightly to give the finished product a smoother look. Set the line color for the rectangle to red (to indicate that it is a cut line) and set the line width to 'hairline'.
Inside that rectangle, type in the desired text in black. You will need to try different font sizes until you find one that fits nicely inside the height of the rectangle. Once you have typed in your text, you need to mirror the text so that the right and left are flipped. This is necessary since we are going to etch the back of the mirrored acrylic, so the text will be seen from the side that is facing down when we are performing the etching.
You will probably need to adjust the width of your rectangle to match the length of your text. I like to try to keep all magnets in a set the same width, but that's not possible if you have text that varies quite a bit in length.
Step 2: Cut the Acrylic
Place the acrylic on the cutter bed so the reflective surface is facing down. You can damage your laser if you attempt to cut with the reflective side facing up.
I prefer to run the etching only at first to assure myself that the text looks good before cutting. It is almost impossible to fix an etch pass that was too shallow once the cutting pass has been made. If the etch left some of the mirrored backing, make a second pass. Do not move the acrylic!
Once satisfied with the etching of the text, run the cutting pass. The acrylic tiles should fall free from the rest of the sheet. If some tiles fall free but others do not, pull the loose pieces out with tweezers before repeating the cut. Take care not to move the acrylic sheet itself or the second pass will destroy any remaining tiles.
Step 3: Attach the Magnetic Tape
Cut a length of tape just barely shorter than the width of your magnet.
If you opted for rounded corners on your magnet, trim the corners of the piece of tape before adhering it to the acrylic or you may accidentally mar the reflective backing.
Wiping off the back of the acrylic with rubbing alcohol can help remove fingerprints and other dirt, helping the tape adhere more securely.
Peel the backing from the tape and press the tape onto the back of the acrylic, making sure to cover all of the etched text.
Pressing the magnet under a heavy book for a few minutes will help prevent the tape from peeling off over time.