The Hayward "flapper" weir has a tab near the hinge that can be compressed with a thin tool and the weir can be removed and the new one inserted. It should take less than 5 minutes.
A skimmer weir is the plastic flap and the opening of the skimmer. It traps leaves that are pulled into the skimmer when the pump is running and then holds the leaves in the skimmer whne the pump shuts down
Drain the pool until the water level reaches two inches below the skimmer faceplate , remove the face plate using a screwdriver , remove the faceplate and skimmer , then replace the gasket and attach the skimmer and then the faceplate
Replace it. The rubber is worn out
Shut the water supply off to the toilet. Flush the toilet. Pull the old flapper out and replace it with the same type. Turn the water back on and test the toilet.
It would be easier to replace with a floating skimmer Check poolrite dragonfly
Is the skimmer plastic? Does it have a black bottom? If so, there is a kit for this. Check out black bottom skimmer repair - search.
If this is an above ground pool you could just pull the old skimmer out cut the rusted section of metal out hit it with some rust converter, replace the section where there is no metal with a new metal forget about the old skimmer setup and replace it with a "floating pool skimmer" they do a better job any way.
Where is it leaking ? Flapper in the tank, replace flapper. Between tank and bowl, separate and replace the seal. Toilet to floor. Replace the wax ring under the toilet, making sure it is thick enough to form a seal and the bolts are installed correctly and holding the toilet down.
Replace the flapper. If that doesn't work, try a Flusher Fixer Kit.
A stuck wier door. That's the the flapper in the skimmer. Or in inground pools you will see twooutlets in the bottom of the skimmer, sometimes one of these outlets are not hooked up to a pipe,so a plug is put in there.If you took out that plug with no line hooked up,water will just dump into the ground.
you may have to prime your pump Clean or replace the filter.
Does the chain or strip that connect the arm to the flapper have a little slack in it? If it is tight, it may be keeping the flapper from seating all the way.