You will need to remove the tank from the bowl. Shut off the water under the toilet and make sure that it is completely off. If you have a ball valve in front of your home at the service into the house, use that valve to shut the water off. Fllush the toilet holding the handle down to remove all of the water. Some water will remain in the bottom of the tank so use a sponge to get rid of that water so your tank is completely dry. Place a bath towel under the angle stop shut off valve under the toilet and unscrew the connector to the bottom of the tank. The towel is to absorb that remaining water in the tank. Loosen the bolts that hold the tank to the bowl. Be careful these fixtures are usually vitreous china and break easily. Remove the tank and place it on the floor on its side so you can unscrew the large nut that holds the Douglas Valve to the tank. It is usually a large plastic nut so you will need a 14" pipe wrench or a slick jawed wrench (without teeth) to unscrew the nut. Replace with a new Douglas valve and tighten it more than snug. Place the foam gasket on it and place it back onto the bowl. Re-install the nuts being careful to tighten each nut the same so the tank comes down onto the bowl the same amount and not putting pressure on one side. This will seat the foam gasket properly. Caution: some tank bolts do not have nuts to hold the bolts to the tank, but extend through the bowl. I would get extra nuts and attach these bolts to the tank, if that is the case. This will give you better results and fewer leaks.
Just take your time but if this seems to daunting, call your local plumber. Hope this helps.
