Underlayment protects the rubber liner from tears caused by roots and rocks. Spread out a large piece so it covers the bottom and sides of the hole and extends up and out of the pond. Spread a large piece of pond liner so that it covers the underlayment. Temporarily secure both pieces with large rocks.
Fill the pond with water so that the underlayment and liner settle into place. Continue filling until the appropriate level is reached. Attach the pump to a water source.
Arrange rocks around the edge of the pond to conceal the underlayment and liner. Trim off any excess. Add water plants and other decorative features to complete the pond.