Anne.....Just add good soil and flowers.
Make plenty of drainage holes in the liner, put in good soil, like garden soil or potting soil and compost, if you have it. Top soil doesn't have the nutrients needed for a healthy garden unless you amend it heavily since it is basically just dirt with a little clay mixed in.
Depending in the depth. If it is deep fill in the bottom with rocks for drainage. Add your soil and plants. These can also be a great opportunity to have a scene in your garden. You can add a theme to the area for example a Zen garden with rocks and a small path, or even a fairy garden if that is your thing.
Fill it with drainage pebbles, then dirt, then flowers!