Step 1: To determine that the flapper is in fact leaking. You can add food coloring to the tank of the toilet. If the food coloring is leaking into the bowl, the flapper is leaking and requires a replacement.
Step 2: Choose the correct flapper for your model of tank.
Step 3: Remove the old flapper by disconnecting it from the flush valve and toilet handle arm.
Step 4: Install the new flapper in the reverse process.
Step 5: Attach the flapper chain to the toilet handle arm. Leave a small amount of stack, so that the flapper remains in contact with the flush valve.
Step 6: Allow the tank to fill and inspect that the new flapper is sealing and flushes properly.