Hand shovel Pencil Utility knife PVC pipe Silicone sealant Vinyl window screen mesh Gravel Plastic hose clamp
Dig out all dirt or rocks from the outside edge of the preformed pond liner where you intent to create the over flow drain point. This should be done a few inches deeper than the depth you want the overflow point to be. Drain the water in the pond to the same point and clean the liner until it is dry with no debris or algae growth on it.
Trace a circle on the outside of the preformed pond liner around the PVC pipe you purchased. Cut this piece out with a sharp utility knife. Slide the PVC pipe into the hole until it is flush against the inside of the pond liner and use silicone sealant to hold it in place. Be sure to seal any possible leaks completely.
Cut two pieces of vinyl or plastic window screen mesh large enough to fit over the ends of the PVC pipe with an inch or two extra on all sides. Put one piece securely over the end of the pipe and hold it on tightly with the plastic hose clamp. You want this end to be removable in case it gets clogged with debris from the pond.
Lay the other piece of screen mesh over the end of the pipe in the preformed pond liner. Secure this with silicone sealant on all sides. This should prevent all aquatic plants and fish from slipping into the overflow pipe.
Surround the end of the overflow drain pipe with gravel or small rocks to both support the pipe and prevent a muddy mess when the pond overflows. Cover the overflow pipe and the gravel with dirt and rocks as before, but leave the end of the pipe open so the water can trickle away. The pipe end can be hidden with a larger flat rock or some plants.