Places like Lowe's actually sell inserts that you can use for a goldfish pond, or you could get a rubber pan from TSC for one. Other than that, you would need something to keep the water in (black plastic thingy), some kind of filter, and some decorations. Add water and fish and you're good to go!
I'd be afraid my ducks would find it, that's why I haven't made one yet
You would also probably need a heater in it, if you planned on keeping the fish in there year-round.
I would put it under a tree, either that or plant some small bushes or something around it. It will get pretty hot if there is no shade over it. Plus, the fish will love love love to eat anything that falls in it. I'd hang a bug zapper over it too, but that's just me
I use a water trough... it works great... had it for 12 years now and my fish keep having babies .. mine is above ground(it used to have rocks around it but moved it)
When we bought this place it came with a small pond with 8 medium goldfish. The filter was a constant pain to keep clean, because of leaves. The dogs, raccoons and possums ate all but one fish.
Never needed a heater for goldfish, they are very hardy. But because of the problems above, it now has an ugly wire fence around it and only an aerator. When this fish dies of old age, the pond will go away. I loved the idea, but didn't realize what a hassle it CAN be.
I am sure there are better and easier ways, but it seemed only prudent to warn you they they can be labor intensive.