First, to tighten a DEWALT miter saw blade, ensure the saw is unplugged. Then, open the motor cover and locate the arbor nut. Using a wrench, turn the nut clockwise until it is tight. Next, close the motor cover and plug in the saw. Finally, turn on the saw and let it run for a few seconds to ensure the blade is secure. If the blade is still loose, repeat the process until it is tight.