The toilet cistern is the part of the toilet that holds the water prior to flushing. In the United States, the common name of this is the tank. The cistern is attached to the bowl by a series of bolts, and there is a gasket between the tank and bowl that keeps the water from leaking out. Over time these gaskets can develop leaks. This is not a major problem, but replacing them will require you to remove and reinstall the toilet cistern.
Ensure that the cistern seal is in place. This is the seal at the bottom of the tank. It is held in place by a large hexagonal nut. If this is a new cistern, this should be in place and ready to go. If you are reinstalling a cistern, now might be a good time to replace this seal.
Slowly lower the cistern onto the back of the toilet bowl. You need to make sure that the outlet gasket fits into the hole in the back of the toilet. Align the two (sometimes three) holes in the bottom of the tank with the bolt holes in the bowl.
Insert the bolts and washers into the holes in the bottom of the cistern. They should poke out and go through both the cistern and the bowl. Again, if this is a reinstallation, you should replace these bolts and washers with new parts.
Tighten the bolts at the base of the cistern. On some models, these will be wingnut type bolts. Tighten them handtight at first. Then use a wrench to tighten one side a turn and then the other.
Attach the water supply to the tank.
Turn the water supply on and allow the cistern to fill. Flush and check for leaks.
Do not overtighten the bolts on the cistern, as this can crack the porcelain.