Drain the preformed pond liner enough to be able to dry off the place where you want the overflow to be. Clean and dry the liner completely, without using toxic chemicals. This is especially important if you are creating the overflow port while the pond is already filled with plants and fish. Dig out some of the dirt or remove the rocks on the outside of the liner in that spot.
Trace around the end of the PVC pipe, which should be no less than 1.5 inches in diameter, directly onto the outside of the pond liner. Use a utility knife to cut the circle out completely. Cut the PVC pipe to the desired length. The outflow end should reach beyond any formal gardens or masonry you have arranged around the pond.
Dig a shallow trench in the dirt where the PVC pipe can lay when it is inserted in the hole in the pond liner. Gently angle the pipe down, which will help the water flow more easily.
Insert the end of the PVC pipe into the hole in the liner so the end is flush with the inside of the liner. Use silicone sealant to attach it firmly in place and to prevent leaks. Apply sealer around both the inside and the outside of the hole for maximum strength.
Cut a small rectangle of vinyl window screen and attach it to the inside of the pond liner over the end of the PVC pipe. This will prevent small pond plants, fish or other debris from flowing out of the pond when it overflows. Cut a larger piece of screening and wrap it over the opposite end of the pipe. Hold this on with a durable hose clamp so it can be removed for cleaning if necessary.
Spread a layer of gravel or small rocks under and around the end of the PVC pipe before burying it with dirt or covering it with rocks or paving stones. This will facilitate even drainage, even when the dirt is exceptionally dry or saturated.
Replace all rock and dirt over the PVC pipe to compliment your landscaping design. Leave the end of the pipe free if it is in a less noticeable place where the water trickling out cannot upset plantings. The pipe and surrounding gravel can also be covered with a flat rock, or the spot can simply be filled with dirt. Then, refill the pond to the correct height with a garden hose.