Now it's time to get your mirrors pointing at each other, and your laser pointing at the mirrors. (You may want to hook up your Arduino via USB and connect your laser to +5 V at this stage for ease of aiming.) This may be the hardest step in the whole instructable! Since I cannot really explain the configuration without effectively copying the NothingLabs instructable, I'll reproduce their explanation here.
"The Y galvo should be positioned with its mirror pivot running horizontally, and the mirror oriented down.
The X galvo should be positioned with its mirror pivot running vertically, and the mirror oriented towards your left.
The laser should be pointed at the mirror of the Y galvo.
The Y galvo should be angled so the laser bounces of its mirror onto the X galvo's mirror.
The X galvo's mirror should be pointed toward where you want the laser show to project."
If my image isn't clear enough, I recommend checking out theirs