Turn off the water supply for the toilet. This is usually located on the wall behind the toilet. Rotate the knob clockwise to shut off the water.
Remove the tank lid. Set it aside in a safe place. Some of these tank lids are very fragile and can shatter.
Flush the toilet and allow the water to run out. You might need to do this a couple of times.
Tighten the lever nut on the inside of the tank, just behind the handle. This nut holds the handle tight in place. Use the wrench to tighten the nut counterclockwise.
Take up any slack in the plunger chain. This is the chain at the end of the handle lever in the tank. When you press the handle, it lifts the lever in the tank, which in turn pulls up the chain which opens the flapper and allows the toilet to flush. If there is too much play in the chain, the toilet tank lever can jiggle quite a bit. Adjust the length by unhooking the chain from the lever and trying another link that is closer to the flapper. Make sure that the flapper can close properly after adjusting.
Turn the water supply back on by rotating the knob counterclockwise. Allow the tank to fill.
Flush the toilet and watch for proper operation. Make any further adjustments in the chain if necessary.
Replace the tank lid.