Choose your design.
Keep in mind that simple outline designs (like the robot and smiley faces) will hurt much less than the solid designs (pacman and space invader). If you can not tolerate the pain, choose a simple line drawing design.
After choosing your design, lay out some painters tape on the bed of the machine, and etch it. This will give you the exact location of the etch, and give you the opportunity to position your body part (hand, arm, foot, etc.) correctly. It will also let you know the duration of the etch, and how long you can expect to endure the pain.
Place your body part in the machine and set the focal point to the skin height. Lasers need a pretty exact focus, so you probably have a window of about +/- one centimeter. Outside of this range and the laser will not be intense enough to burn the skin.
Show off your cool burnt skin tattoos to random people and watch them recoil in horror and disgust.