Fixing a leaky toilet tank is one of the most common plumbing repair jobs that you can DIY. Most of the time, you can do it by replacing the gaskets placed around the tank bolts and the spud washer, which are components that can deteriorate and fail over time due to minerals in hard water. Before you start anything, you should secure necessary tools and materials, including an adjustable wrench, screwdriver, sponge, small wire brush, rag, white vinegar and, of course, a new spud washer and replacement tank bolt gaskets. Now, follow these steps:

1. Turn off the water supply by turning clockwise the shutoff valve that is found below the tank.

2. Drain the tank completely by flushing the toilet and soaking up any remaining water in it using a sponge.

3. Hold one of the tank bolt nuts located on the underside of the tank using an adjustable wrench.

4. Loosen and unscrew the tank bolts inside the tank with a screwdriver.

5. Remove the tank from the bowl and lay it down in a safe place inside the bathroom or in another area of your house. You might need help from another person for this step.

6. Take out the old spud washer, and if it does not budge, try using your wrench to get it to turn.

7. Insert the new spud washer and tighten it over the spud nut.

8. Remove old bolts and gaskets and then clean the areas around the holes on the tank. To scrape off any old gaskets that might remain stuck inside and outside the tank, you can use the wire brush. Remember that you need to make sure the surface on which the new gasket will seat is clean for a proper seal.

9. Place the new gaskets onto the bolts and see to it that they are firmly seated.

10. Insert the bolts through the holes in the inside part of the bottom of the tank. On the underside, reinstall the nuts onto the bolts, gently tightening them by hand until they are snug.

11. Re-install the tank on the bowl. Alternating side to side, tighten the bolts and nuts until the gaskets look like they are evenly and properly seated. For this step, remember not to over tighten the bolts to avoid cracking the tank.

Aside from these general steps, your repair plan for a leaky toilet tank can also focus on the area where the leak specifically occurs. Typically, water leaks springs from the water supply line, tank-bowl assembly and the flapper. If one of these is your situation, then you can fix it with just a few inexpensive parts. For the tools, you will need the following: adjustable wrench, ball-cock assembly, bolt assemblies, tank-to-bowl gasket and sponge.

Fixing the Water Supply Line

Check the area where the water supply line meets the tank, as well as the line itself. Using your fingers, test for drips.
Check if the jamb nut holding the water supply line to the tank is loose; if so, tighten it using an adjustable wrench, but do it carefully. Keep in mind that nut and fitting is usually made of nylon, which means that they can strip when over-tightened. The nut should be snug only about a quarter turn past hand tightening. Also, porcelain tanks are also susceptible to cracking due to the pressure of over-tightened nuts.
If tightening the nut did not work, you can try replacing the ball-cock assembly. A ball cock is the water supply valve and float arm that you will find inside the tank.
Cut off the water supply and flush the toilet to allow water to escape from the tank. Using a sponge, you can mop up any of the remaining water.
Loosening the jamb nut again, pull out the old ball cock and put in the replacement. Tighten the new jamb nut and fitting, but make sure you do not overdo it.
Turn the water supply back on to allow the tank to fill and see if there is still a leak.

Fixing the Tank-Bowl Assembly

Check the bolts holding the toilet tank to the bowl and see if the nuts holding them are loose. If so, you need to tighten them and see if this stops the leak.
If the previous step did not work, you need to replace the gasket sealing the connection between the bowl and the tank. This gasket loses elasticity, hardens and cracks over time.
Turn off the water supply and then flush the toilet to get rid of the water in the tank. You can mop up the remaining water with a sponge.
Loosen the nuts securing the hold-down bolts. In doing this, you might need to hold the top of the bolt with a screwdriver while loosening the nuts with the wrench. Then, loosen the jamb nut securing the water supply line to the tank.
Free the tank from the bowl and then remove the old gasket and replace it with the new one.
Re-install and secure the tank back on the bowl by inserting and tightening the hold-down bolts and washers. Again, avoid over-tightening them to prevent some components from getting damaged.
Turn the water supply back on to allow the tank to fill and see if there is still a leak.

Fixing a Leaking Flapper

Test if the flapper is no longer seating correctly. The flapper is the component located at the bottom of the tank that prevents the flow of water into the bowl until you flush the toilet. You can determine a leaking flapper by squeezing a few drops of food coloring into the tank and seeing the color showing up in the bowl without flushing. Also, if it has hardened, it should be replaced.
Turn off the water supply, flush the toilet and get rid of any remaining water with a sponge.
Snap off the old flapper and remove the chain leading to the flush handle. Place the new flapper and re-attach the chain.
Turn the water supply back on to allow the tank to fill and see if there is still a leak.

When the tank itself has cracked causing the leak, the only solution is to replace it entirely.