Turn off your Glowforge and then open the front door and carefully remove the crumb tray.
Without removing the carriage plate and belt, locate the black pulley fastened by a metal screw on the right end of the laser arm.
Insert the short side of the 3mm hex L-key into the metal screw with the long handle pointing toward the front of the unit.
Turn the key clockwise to loosen the screw until the pulley can slide with some friction. Don’t remove the key from the screw.
Holding the key with your one hand, position your other hand to push the pulley to the right until the belt is fully extended.
While keeping tension with your other hand, turn the hex key counter-clockwise to tighten the screw in the pulley.
When pressing lightly on one side of the belt, the other side should not move. It may take a few tries to get the belt tension correct. If the belt has too much slack, loosen the pulley and try again.