Garden ponds are usually best constructed with a high quality epalyn or butyl rubber pond liner. These liners are flexible, tough, long-lasting and easy to shape to the hole. When creating a natural wildlife pond you may want to consider a layering approach when installing the liners with a protective underlay above and below the pond liner. This approach will help to achieve a natural look while also protecting the pond liner from leaks.
To achieve the natural pond look, first add a layer of sand to the bottom of the pond for added protection against possible damage caused to liners by stones. Once you have firmed down the sand, lay-down the protective pond underlay, then the pond liner and a top layer of Geotextile protective underlay. The reason for adding the top layer of underlay is that it allows the pond to be back filled with about 3 inches (about 8cm) of soil giving a natural appearance by hiding the liner. The soil also allows you to plant marginal plants.
Once you have back-filled your pond with soil you can fill it with water. Rainwater is preferable, so either wait for the rain to do the job for you or fill from a rain-butt.