Flexible liner usually consists of 45 mil rubber and allows the builder to be more creative.  Future additions are also possible with flexible liner.  First, excavate the hole down to at least 18”, if fish and water lilies are to survive the winter. Incorporate shallow ledges 8-10” deep for marginal plants if desired.  Next, make sure the edges are level and remove stones or sharp objects and smooth the bottom.  It is a good idea to apply a shallow layer of mortar along the edges to prevent settling. Apply a 2-3” layer of sand on the bottom and ledges to prevent punctures.  Pond underlayment is also an option and is recommended. After laying out the rubber liner, start filling it with water, pulling out the wrinkles as it fills.  Place stones around the edges to help hold it in place.  Water garden edging is a product that secures and hides the liner, avoiding the need for stones at all.  Samples are available for observation.  Finally, apply gravel and stones on the bottom to disguise the liner, if desired.  Avoid using limestone gravel as it raises pH.  Concrete ponds typically crack in our area if not drained in the winter.  They also leach lime in the water, raising the pH, and are not recommended as water gardens for these reasons.