Unplug the saw. With your Allen wrench, loosen the mounting bolt that holds the blade to open the slot that the blade sits in. You'll see a little metal peg within the slot.
Slide the bottom of the blade into the slot, positioning the small hole that's on the bottom of the blade so that it hooks on the peg. The teeth of the blade should be facing away from you as you hold the saw out in front of you. Tighten the mounting screw to secure the blade.
Put on your goggles. Position the saw just above the marked line to be cut. Have one hand on the trigger and the other hand gripping the middle-front of the saw, at the rubberized part of the body.
Squeeze and hold the trigger to start the motion of the blade. Bring the blade down to the wood, setting it gently on your line. Apply steady downward pressure with the hand that's gripping the saw to push it through. Slide the saw forward or back as needed to make your cut.
Pull the blade out and away from the wood before you the release the trigger. (If the saw stops in mid-cut, the blade will get stuck in the wood.) Release the trigger and keep the saw held up and away from you until the motion of the blade has completely stopped.