There are lots of ways of making ponds and water effects, and this little project describes just one.
Cut a hole in your base to give the pond some real depth. This is easier if you add a layer of corrugated cardboard to your base with the pond hole already cut out. Fashion the pond banks from air-drying clay. Make them overhanging here and there.
Pour a small amount of PVA glue into the pond. Allow it to dry. This is to get a smooth, water-like finish. Paint the pond green, with swirls and splashes of darker green. Paint it darker near the banks. Varnish, when dry, with a gloss varnish. Paint more washy green on top of the varnish, leaving some of your first layer green showing. When dry, varnish again. Repeat as many times as you can be bothered. The more layers, the better the water effect. Add rushes and grasses - see the grasses section for details.