Installing a new riding lawn mower blade isn't a complicated procedure. Make sure, first, that you have the proper blade by checking your lawn mower owner's manual for the part number of the blade. And remember, safety is of utmost importance when you are working on lawn mowers; if you fail to remove a power source or keep the machine level, you risk losing a finger. Keep safety first and follow proper protocol in order to install a blade on your riding lawn mower.
Park the machine on flat, level ground. Put the parking brake on so that the machine will not roll while you are working on it.
Turn the machine off. Remove the key and disconnect the power to the battery by removing the negative cable from the battery. Be very cautious: if you touch the positive battery cable to anything that is grounded, it may spark and cause an explosion.
Get your tools ready: a ratchet, socket, and a breaker bar, and a pair of channel lock vise grips or a piece of wood.
Push the blade up on the screw that extends downward under the lawn mower deck. Secure it with the vise grips or with a piece of wood wedged in between the blade and the deck in order to keep the blade from spinning while you are tightening the nut.
Slide the nut onto the screw and tighten it with the breaker bar on the ratchet. Most blades are sufficiently tight at 60 to 100 foot-pounds of pressure. You can push the breaker bar at about 1 foot distance from the nut and you will need to exert about 50 pounds of pressure to tighten the blade. If you don't have a torque wrench to measure the foot-pounds precisely, think of the pressure in terms of your body weight: push with 1/3 of your body weight if you weigh 150 pounds or 1/4 of your body weight if you weight 200 pounds.