I have a simply made and quite small pond on my allotment. It is not deep, it cannot be, for I dug it in the fruit tree section of my plot and tree roots got in the way of my digging too far down. I constructed it out of a tarpaulin, a hole and obviously water, along with a dollop of frogspawn that I managed to acquire one Spring some years ago, and I have had frogs every year since then. The frogs, which occasionally surprise visitors by leaping out, serve an important function of eating slugs that would otherwise attack my crops. But you hardly see the frogs, for they hunt in the compost heap, the cabbage beds and the greenhouses during the day. However,they delighted my three year old grand-daughter when she visited and saw a large, fat frog. I do think, though, that once we were visited by a heron that was too heavy for the tree branch on which it perched as it searched for frogs, and consequently broke it.
The hole can be simple, but it can also be of differing depths to cater for a variety of plants, but it must be watertight. In the past farmers digging dew ponds used to use puddled clay, which is clay that has been soaked and beaten into a paste that is smeared around the base and sides of the pond and allowed to set solid. It's watertight, but messy. You can use concrete, but the simple and effective way is to use a pond liner.
First ensure that the hole has no sharp stones that can puncture the liner. Doing this involves clearing bottom and sides of any debris, however small. Some pond makers add a layer of fine [not gritty] sand, but this does not protect the sides. Then begin with a piece of pond underlay. To determine the area needed work out the total area of pond and sides and double it. This is the area of underlay that you need. It will extend over the sides. Smooth it out and press it down into the corners of the pond. Then take exactly the same sized piece of butyl pond liner, laying it down to cover the underlay. The exposed area of liner is then covered with soil or gravel to landscape the feature. Soil is preferable as it allows pondside plants to grow, and this creates a more natural pond. You can then fill up your pond.
Here is an essential word of warning. If you are installing an electrical pump, get it installed by a properly qualified electrician. Electricity and water are a dangerous combination.
Another word of warning. Children. Ponds are dangerous for young children, as it is possible to drown in an inch or less of water. I put a safety grill over part of my pond, allowing enough space for the frogs to come and go. When children visit the pond they are always supervised.