When using pondliner, we suggest first laying underlayment fabric as a protective barrier between the bare ground and the liner. Other ideas may include sand or old carpet (watch for staples).
Lay out your liner in the hole and push the liner into every space possible. Any large sections of leftover liner, such as corners, may be cut off and added to the center to act as an additional protective layer on the inside. However don’t do a final cutting yet until the water feature is completely full of water and finished.
It’s a good idea to add another layer of fabric on the inside of the feature as additional protection for the liner.