The first thing to do is to isolate the water supply. Now you can drain the cistern of water. Now, flush the WC and clean out any water that remains in the cistern. Unscrew the holding bolts that hold the WC cistern to the pan. Take the chain off the handle. Clapham Plumbers carry all the right tools for all plumbing work. Remove the cistern and place it on the floor on some old towels. Take off the large rubber spud washer and nut from the bottom of the flush valve with a wrench. Now, remove the old flush valve and put the replacement valve in the valve hole and make sure that top of the overflow pipe is at least one inch below the cistern high level indicator and the aperture where the flush handle is fitted. Put the flushing valve underneath the lever arm for the handle. Secure it to the cistern from below using the spud nut. Tighten the nut slightly more than hand tight using a spud wrench or channel type pliers. Try not to crack the cistern as you tighten any of these fittings. Put the replacement spud washer over its nut, small side facing down. Place the cistern face up and put the correct washers on the tank bolts and insert it into the holes from inside the cistern. Put the brass washers and hexagonal nuts onto the bolts from below and tighten them slightly more hand tight. Place the cistern over the bowl and sit it down so the washer sits over the water inlet in the pan and the cistern bolts fit through the holes in the pan flange. Fit the cistern and bowl to each other. Or, you could ask a Clapham Plumber to carry out the work. Fit the cistern and pan together. Re-connect the water supply at the fill valve inlet. Re-connect the chain to the handle. Attach the refill tube back to the top of the overflow pipe. Turn on the water supply at the stop valve and test the flush. Check for any leaks.