A water garden should be sited where it will be enjoyed the most. Sun exposure should be considered if water lilies are wanted, as they need 5-6 hours of direct sun in order to bloom their best. Places where runoff may enter the pond should be avoided because they may carry lawn pesticides and fertilizer. Any place where nothing else will grow is a prime candidate. There are two main types of liners for water gardens, rigid plastic and flexible rubber.
Rigid types are easy to install, but are typically small, and may require re-lifting every few years as the edges tend to settle. They simply require an area to be excavated to their shape. Add sand to the bottom only if it cannot be smoothed out due to rocks or gravel.
Make sure the edges are level and the substrate of the hole is packed tight before filling with water. Stones or groundcover may be used to disguise the edges.