Here is my pond....
I have a thick card 'ground' level on top of the plywood base.
To give a little more 3 dimension 1st cut out the shape of the village green and the shape of the pond then draw round pond onto the base card and cut a square slightly bigger. Cut a square of Perspex or similar transparent plastic to fit the square hole.
I then painted a gungy green/brown mix onto the ply base, lighter at the edge and darker in the middle. I then made a wall of plasticene/bluetac around the edge of the perspex and poured in a mix of clear varnish and Humbrol MATT 80 Matt Grass and let it set.
Feather the edge of the pond with a craft knife and paint MATT 23 Dark Earth close to the waters edge and MATT 121 Pale Stone (or similar sandy colour) on the outside and blend them in.
Place varnished square into hole and glue village green in place.
Although you cannot see it too well on these pictures i added some contours to the green also before the flock (Gaugemaster GM173 Flock Meadow Green).