There are several ways.

If you want it to last a long time, you can build a wall out of cinderblock concrete and rebar. Pad the inside of the pond wall with underlayment or carpet and place your liner in. You then can veneer the outside wall with stone or brick and cap with stone or brick to cover the top of the liner.

If the wall will not be very tall, you could use an interlocking decorative block that can be found at the big box stores and add the liner and cap it off.

Our pond is on a slope so we had to build a large 3 foot retaining wall out of concrete block and then veneer it.

Hope this helps. I know someone must have pics using an interlocking block system.

LSST

I've made 3 smaller "patio ponds" out of various materials that I have scrounged up. Depending upon the size of pond you want to create, some of these techniques may be useful to you. 2 of my ponds get dismantled for winter and the fish are overwintered inside the house in a large aquarium.

My smallest pond is made from wood shipping pallets that I got from work and a shower curtain liner. I measured the liner and decided on a size and depth to cut the wood to. I built a basic solid box out of 2x4s that was approximately 2' tall and 4'x4'. I finished the top edge with a 2x6 and stained it. Each spring I go buy a new curtain liner and staple it to the inside edge. Add water and a pump and you're finished.

The 2nd pond is built in a somewhat similar fashion, with lumber I repurposed after tearing out an old deck. I built 8 boxes (about 2'x2'x2') that served as the 8 angles of my octagon. I then attached 2x6s between the corners of each set of boxes, creating one large octagon. I bought a huge roll of the thickest plastic drop cloth I could find, lined the inside twice, and stapled the edges. The 8 boxes were filled with potting mix and planted with flowers. I added a pump and fountain, and it works great. Again, this gets dismantled in the fall, and the fish are brought inside. It's so shallow it will freeze solid otherwise. New liner every year, also.

My third pond is built out of chunks of concrete torn out when we re-did the sidewalks. This one is permanent. It sits on dirt, so I did dig down just a little bit to increase the depth of the pond and also to get the lowest layer of concrete below ground level. I just mortared the concrete chunks together as I built the wall up to the desired height. Once I was close to the desired height, I put the pond liner in (yes, I actually spent money on a REAL pond liner. LOL!) and then placed more concrete chunks on top of the edges of the liner to hide it and give it a more finished look. I looked up a concoction containing yogurt and ground up moss on the internet, made it and covered the concrete with it. It's starting to grow a little bit, which gives it all a more weathered look. Add water, fish, and a pump.....Viola!

Those are my creations! Other than the pumps, I really have a minimal amount of money invested in each. They aren't award-winning by any means, but I think they add a lot to my otherwise bare patio areas.

My MIL's pond is above ground built out of large stackable blocks (no mortar) with a liner. The liner is held in place with capstones.

One drawback of my MIL's pond is that the water gets too warm (approaching 90F) in the summer time, so you might have to shade it depending on what you put into your pond. Our pond is half above and half below ground, so it stays pretty warm in winter and pretty cool in summer.

traschuesler emailed me and asked me how many blocks tall my MIL's pond is. My answer is that it is at least 48"--since the blocks are 8" tall, that makes it at least 6 blocks tall. The blocks are large--they must be at least 75 lbs each. Water content is about 2,000 gallons.

The blocks are stackable no-mortar blocks used specifically to build mortarless retaining walls. They are not cinder blocks (the kind used with mortar to build walls).

Here is a link that might be useful: An example of mortarless blocks

hi
Have one using stacked 4x4's nailed together. Limits you to square shapes though. I used 5 layers so about 18 inches above ground and dug down about 20 inches below ground. Allows you to sit on the edge. and the extra depth does help keep the heat down. It does not keep toads out .lol
A curved one will reguire a lot more thought lol. gary

I have one made of pressure treated 2 x 12's. Isn't as pretty as blocks though.

I was able to use a couple of old pool liners and some railroad cross ties. the main thing was to brace up the sloping end but we went ahead and sured up the other sides as well. I used a sunk pump as an areorater for the fish and just attached a timer to the pump