Step 2: Making the Drill Guide
I happened to have a couple 3/4" thick scraps already glued together from a previous box joint project which was convenient. But starting from scratch, you'll want to cut your pieces 2 1/2" x 4 3/4" and 3 1/4" x 4 3/4" if using 3/4" material. If you have precut mirrors, you'll want to cut your boards to match your mirrors. I like the 3/4" board because it gives the guide a bit more weight and stands nicely. The measurements aren't exact so use the odds & ends you have lying around. Even if you've burned some holes in 'em like I did, they'll still work just fine. Attach the 2 wood pieces together at 90° with glue and a couple screws.
I wasn't able to take pics while cutting the mirrors, but it's very easy on thin glass. Simply dip the cutter wheel in oil and then run it along a straight edge lined up on your mark. If it makes a scratchy "fingers on the chalkboard" sound, you doing great. Light pressure is all it takes to score the mirror which can then be snapped off. When breaking it, I like to support one side of the glass on my workbench top with the scoreline even with the edge of the bench. This makes it easy to break the piece by deflecting the other side downward. It is always a good idea to wear leather gloves & eye protection when cutting glass and mirror. Remember that one mirror needs to be slightly narrower to accommodate the width of the other mirror (i.e. 2 1/2" wide and 2 3/8" wide if using 1/8" mirror).
Once your glue has dried and your mirrors are cut, attach the mirrors using double stick carpet tape or an adhesive.