Turn off the water that fills the toilet. The valve is usually found on the floor behind the toilet. Turn the valve all the way clockwise until it is snug. Flush the toilet to remove the water from the tank.
Open the tank cover and place a towel into the tank to soak up any of the residual water that did not drain.
Remove the mounting bolts holding the tank to the bowl There are usually two, and they can be accessed under the tank. Using an adjustable wrench, turn each nut counterclockwise until it falls off. Lift out the bolt from inside the tank.
Pull the tank straight up to remove it. Take out the old rubber washer that sat between the tank and bowl. Wipe down the porcelain with a dry cloth or paper towel. Set the new spud washer with the indentations facing upward, into the opening on the bowl.
Install the new tank by setting it on top of the spud washer and pressing it firmly in place. It will still wobble a little but when you tighten the bolts, the rubber will form to the shape of the tank and form a tight seal.
You should have four rubber washers that will keep the metal bolts and nuts from touching the ceramic. Place a rubber washer inside the tank over the screw hole. Slide the bolt through. Place a rubber washer under the tank over the bolt. Screw on the wing nut on the bottom of the tank.
Repeat Step 6 for the second bolt. Both wing nuts should be tightened to the same pressure, but not too tight. Tightening these nuts too much can crack the tank. The tank should remain firmly in place without wobbling.
Turn the water valve back on behind the toilet. Let the tank fill with water and then flush it. There should be no seepage or leaks if the tank was installed properly.