The first and most critical step is to prevent any water flow to the toilet. Locate the water supply shut-off valve, typically on the wall behind the toilet. Turn the valve clockwise until it stops. Next, remove the tank lid and flush the toilet, holding the handle down until as much water as possible has drained from the tank. There will be about an inch of water left in the bottom of the tank. Use a large sponge or absorbent towels to soak up all the remaining water from inside the tank. Place a bucket nearby to wring out the sponge. Ensure the tank is completely dry inside before proceeding.

Locate the water supply line, which is the flexible hose connecting the shut-off valve to the bottom of the toilet tank. Place an old towel or a small bowl directly underneath the connection point at the tank to catch any trapped water. Using an adjustable wrench or channel-lock pliers, grip the coupling nut on the supply line where it threads onto the fill valve shank. Turn the nut counter-clockwise to loosen and unscrew it completely. Let any remaining water drain out onto the towel.

Look underneath the toilet bowl platform where the tank sits. You will see two or three nuts securing the tank-to-bowl bolts. To remove them, you'll need to work both inside and outside the tank simultaneously. Place a flathead screwdriver on the head of one of the bolts inside the tank to keep it from spinning. While holding it steady, use a wrench or a deep socket to turn the corresponding nut underneath the bowl counter-clockwise. Once the nut is removed, repeat for the other bolt. After both bolts are undone, carefully grip the tank on both sides, lift it straight up, and off the bowl. Place the tank gently on a protected surface, like a piece of cardboard or a stack of old towels, to prevent damage.

With the tank removed, you can now access all the old components. Pull off the large, spongy gasket (often called a spud washer) from the threaded flush valve outlet on the bottom of the tank. Then, pull the old bolts and their rubber washers out from inside the tank. Using a rag and maybe a plastic scraper, thoroughly clean the area on the bottom of the tank around the flush valve outlet and the corresponding surface on the toilet bowl where the gasket sits. Any dirt or residue can prevent a good seal. Take your new bolts from the tank-to-bowl kit, place a new rubber washer on each, and insert them down through the holes from inside the tank. Finally, press the new large tank-to-bowl gasket firmly onto the flush valve outlet on the tank's bottom.

Carefully lift the tank and position it back onto the bowl. Look down through the tank to align the new bolts with the corresponding holes in the back of the toilet bowl. Gently lower the tank until it rests on the bowl, with the new gasket compressed between them. From underneath the bowl, place a metal washer and then a nut onto each bolt. Hand-tighten both nuts until they are snug. Now, using your wrench, begin to tighten the nuts a half-turn at a time, alternating between the left and right sides. This ensures even pressure. Continue until the tank is stable and does not wobble or rock. It should feel secure, but do not apply excessive force.

Reconnect the water supply line to the fill valve shank on the bottom of the tank. Thread the coupling nut on by hand until it is snug. Use your adjustable wrench to give it a final quarter-turn to ensure a tight seal, but do not overtighten. Now, slowly turn the water supply valve counter-clockwise to turn the water back on. Watch the connections closely as the tank fills. Look for any drips from the supply line nut or from the new tank-to-bowl bolts. Once the tank is full, flush the toilet several times. After each flush, use a dry paper towel to wipe around the bolts and the seam between the tank and bowl to check for any moisture, which would indicate a leak.