A skimmer is a place where water overflows into a container and then into a pump which pumps the water back into the pond. The key to a skimmer is hinged door that allows just a thin sheet of water to flow into the container. This causes surface tension in the water to pull floating debris into the container where it's trapped in a basket.
For years I was not a big believer in skimmers. I'd had one on my swimming pool and it didn't get all the leaves. I figured with a pond's edge being even more irregular, plants in the pond and other structures that even fewer leaves would be removed. I was right about the leaves, but there were other things I hadn't considered. Dust can be blown into the pond and sit on the water's surface and make the pond look pretty bad. There's also string algae that can break off and float on the surface. I've since become a believer.
You can buy skimmers which I'm sure are very nice. One thing I have seen that I don't like is combo skimmer/filters. Actually I don't like any "combo" things for the pond. Seems to always mean "does neither very well". Skimmers should have a basket that's easy to remove, clean and replace. I don't want to move pads, bio balls and other stuff to clean the skimmer basket.
Where to place the skimmer so it functions at peak efficiency so every bit of floating matter is whisked away at amazing speed? You need to take into account normal wind direction for your area, trees, water flow, swimming fish and butterflies flapping their wings in China. Punch all that data into a super computer and you'll have your answer. OR put it where it's best for you. Out of the main viewing area and where it's easy to clean. Forget about wind, water flow, etc... It doesn't really make that big a difference.
Making Your Own Skimmer
My skimmer was not very well planned out. But here's how it was done and improvement suggestions.
I framed a box using concrete blocks, 2 courses deep. Just dry stack the block and fill with concrete. The pond liner was laid in the pond and inside the skimmer box. I didn't have any seam, it was continuous liner. Having the skimmer in the corner of a round side meant no extra liner was needed for the skimmer.
I put a 2" thick layer of concrete over the liner in the box. The liner in this small a box has some large folds, the concrete covers these. Also because I'm going to be removing and inserting the basket for cleaning I didn't want to worry about wear on the liner.
The basket frame was made from 3/4" PVC pipe and fittings and covered with shade cloth. Wish I'd... made the skimmer basket out stainless steel or rigid plastic, for easier cleaning.
The spillway, where water goes from the pond into the skimmer, was covered with mortar and formed into fake rock. But the actual spillway had straight sides. Wish I'd... made the spillway lower in the pond. Currently if the water drops more than 3" inches the skimmer goes dry.
I totally got lucky on the skimmer door. This can be the trickiest part of a skimmer. I used a piece of corrugated plastic. This material is very much like corrugated cardboard. About 1/4" thick and hollow inside. It's used for lots of things including inexpensive signs and U.S. Mail bins. I cut a piece the same width as the spillway and scored one side so it folded in half easily. I trimmed one half to be 1/16" less wide than the bottom. I pushed the bottom half into the spillway and it stayed in place just from being tight against the sides. This corrugated plastic is just a bit lighter than water so the top half floats to the surface water.