Turn off the water supply valve behind the toilet
Remove the tank lid
Check the flapper for wear, warping, or misalignment
Reposition the flapper so it seals properly
Replace the flapper if it is damaged
Check the chain length and adjust it so it is not too tight or too loose
Inspect the fill valve for damage or continuous flow
Adjust the fill valve float to the correct water level
Replace the fill valve if it is faulty
Check the overflow tube for excess water
Lower the water level if it is set too high
Inspect the flush handle and lift arm for sticking
Make sure the refill tube is not inserted too far into the overflow tube
Replace worn tank parts if needed
Turn the water supply back on
Test the toilet for leaks and proper shutoff