A back yard pond is a wonderful source of relaxation and enjoyment and they are not at all hard to make. I have about a thousand gallon pond that is several years old and it is a delight that consumes very little time or money. Although mine is made of concrete the flexible liners are probably just as good.
Dig your hole in an interesting shape. Avoid sharp corners. Have some deep parts and shallow parts; shallow shelves near the edges and deeper in the middle is okay. The deep parts give temperature stability and a place for sediment to collect. My pond has a partition wall separating it into two ponds connected by a hole in the wall.
Put in lots of rocks. The surface area of the rocks gives animals and plants a place to grow. Conceal the edge of the pond with rocks if possible.
Plants are good. Find local plants in natural ponds and buy more if you need to. I was lucky to have some nice water lillies given to me. You will have to remove excess plant growth every now and then.
Here in Florida it is essential to put in some mosquito fish. They are little live bearing gambuzia that reproduce quickly and keep the surface from forming a film. The rivers and lakes are full of them; a little bread in a jar underwater will attract enough to start a seed population.
Little gold feeder fish are sometimes as cheap as ten for a dollar. Buy several dozen; if the pond is much cooler or warmer than the water they're in let the temperature equalize for a while before you dump them in. You will lose about half of them in the first week and half of the remainder in the next few weeks but the survivors will begin to grow. Feed them old fashioned oat meal; it's a lot cheaper than fish food. The originally tiny feeder fish will gradually grow to be big and tame.
I don't filter or recirculate the water. A thin high fountain adds water to the pond. Municipal water has lots of chlorine and no oxygen so if you put too much in too quickly it will kill all the fish. The thin high fountain allows the water to lose chlorine and gain oxygen; the total flow is small enough that you can just let it overflow for the surrounding plants. Plants nearby will drop roots into the pond and suck out the water so you will have to keep adding.