First, build yourself a hefty base. I made the pond 8x4' because you can pretty much buy everything at the perfect size or only will need a few simple cuts.
If you want, you could probably build it right on the floor. You could also use cinder blocks as the exterior framework.
I built the frame out of 2x12's, as they were all pre-cut and very solid for a frame. They are also fairly inexpensive as well. I put a $7 sheet of paneling underneath the frame, and put support nails every foot or so, to re-inforce the 2x12's from bowing out..
After the frame was well attached, we put in the soft pre-liner..
(Ignore the very ugly room in my new house, LOL, no time to pull the panelling yet.. Fish first, house later! )
Then we put in the pond liner.. It's the extra heavy duty kind.. You could probably go with a lighter one, but this was left over from a previous pond. We cut off the excess..
We also decided to get tricky and put in a bulkhead and overflow on the bottom..
Then we added the trim around the outside.. Some nice fencing for very inexpensive.. A little sanding and it came out very nice. This helps re-inforce the 2x12's from bowing.
And then you have a finished product! Kind of..
The best part is yet to come, when I put a 'shell' around the outside of the wood..I can use whatever color and texture I want, and cut it to fit. I have a million of options.. I will also design a nice looking lid for it to hold the humidity in and create shade, but I will have an air exchanger anyways.