Turn off the water supply to the toilet tank. The shutoff valve is at the bottom left side of the tank in most cases.
Flush the toilet to drain the tank.
Soak up the leftover water with sponges and deposit the water in a bucket or directly into the toilet bowl. Continue soaking up the water until all of the water is removed from the tank.
Blot the inside of the tank with paper towels.
Remove the old flapper, tank ball and accessories like the chain and links. Look at the existing valve seat to determine what it's made out of. It will usually either be brass or plastic.
Clean the existing seat, using the nylon scrubber if it is plastic or the steel wool if it is brass. A wet/dry sanding block will also work on a brass seat. Use paper towels to thoroughly clean and dry the seat once more. Determine the position of the new valve seat, reading the instructions that came with your repair kit.
Peel away the protective layer of paper from the bottom side of the sealant ring and set the ring in place according to your instruction sheet. Press the ring down firmly against the seat so it creates a good seal.
Remove the top layer of protective film from the ring and set the new valve seat on top of the sealant ring. Apply firm, even pressure to create a good seal.
Install the new flapper and chain as directed and turn the water supply valve back on so the tank can fill up.
Flush the toilet and listen and look for any signs of leaking tank water.