Sand is plentiful and inexpensive in most home and garden stores. Use 2 inches of builders sand to cover the bottom of your pond liner.
Hopefully, the design of your pond has tapered sides, slightly sloping so you can build up sand along the outside edges. You want the sand deep enough so that there is no chance of the liner showing through over time.
Ornamental rocks add a nice look and a practical resting place for fish and other marine life. Set some on the shelves in the pond under the water and set up a couple so they stick up above the surface of the pond.
Birds will use this as a standing point to get water. Add flat rocks around the top edge of the pond so that they protrude and hide the upper edge of the liner.
Water plants will obscure the sides of the liner where sand might not stay. Water lilies will float on the surface but reeds like cattails will root in shallow areas.
Check with your local extension office to find which plants they suggest for growing in your pond since different environments are conducive to specific plants. Situate perennials to grow around the top and hang over the edge naturally.
Hostas, daylilies, coleus and even mints grow nicely by the water’s edge.
Construct a mini wooden pier to extend out over the edge of the pond slightly. It will hide the liner under it and you will be able to use it to pull leaves out of the pond in the fall.
Anchor it firmly in the ground outside the pond area so there is no chance of falling into the pond while standing on it.