This pond was built in December 2007 in the earlier days of my turn-the-front-lawn-into-garden project.
The tires came from a park mowing tractor so are wider than normal tractor tires, giving extra height to the pond. You can usually pick these up free, in New Zealand anyway, from large tire repair and supply workshops: mine came from the parks maintenance company I work for - free disposal for them and free supply for me!
two tractor tires (one, two, tractor, car - your choice)
large cardboard box, old carpet, etc
thick polythene sheet from a local BORG (Big Orange Retail Giant)
flexible drainage coil (I had this lying around)
geotextile (eg weedmat, mudstop, etc) [oh, or you could use old carpet!]
soil, container mix, or compost
water plants and land plants
two terracotta pots
reciprocating saw, jigsaw, hacksaw or even a sharp sturdy knife
short length of 25mm PVC pipe
Step 1: Cut sidewalls from tires
Cut one sidewall from one tire and both walls from the other, leaving about a 75mm (3 inch) lip. I only cut out the upper wall of the second tire at first, but the polythene would not mould around the convoluted 3D-ness of this arrangement. Unfortunately, I did not take photos of the second try, but you'll get the idea.
Smaller tires can be done with a jigsaw or even a small knife: see the paring knife I used on car tires (thanks for the use Sweetie ... oh, I thought I'd asked you about that?... oh ... er ...).