If you didn't skip to this step from step 4, you'll have to use a really long drillbit and a hand drill.
Drill the larger strip of cutting mat so that the laser just fits through. I used a sheet metal step drill to hand-do the last little bit to ensure it's tight.
Drill the smaller strip so that the laser can sit comfortably.
Work the lasers into the holes. Glue the base if necessary (but don't glue both parts of the laser!). If your big cutting mat strip happens to be at button-height on the laser, then you can turn the lasers on and off by sliding them in and out of the holes. If it's not, electrical tape the button down and unscrew the battery compartment until it turns off.