Step 6: Safety!
I stoopidly forgot that the laser beam was completely exposed on the left side before it hit the final mirror.
Shortly after the space invader etch started, I noticed a lot of smoke. This smoke was not coming from the etch, but from the left side of the cutter, where the upper part of my left arm was resting. When I looked over, I saw that my shirt was smoldering.
I was so focused on keeping my forearm in place (as to not ruin the tattoo), I did not notice that the laser was no longer etching it, but rather cutting a gash into my upper left arm.
Needless to say, I only etched my right arm from that point on.