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I dug one about two years ago. I think I broadly followed the wildlife trusts guide.
It's quite small, about 1.2m by 80cm. I used an underlay as well as a water proof pond liner because some guides recommended that. Not sure how necessary the underlay was, but didn't add much to the cost, the liner is the expensive bit.
It's been great, has attracted some frogs and the birds like having a bath in it.
These guys are great for native plants. They tell you which ones are oxygenating, how deep they should go etc.
No experience. But Charlie Dymock (sp?) knows a lot about ponds if you're doing any YouTube searching for inspiration.
I can't remember the tip, but she had a really good one on getting a natural transition. To me that's the key difference between nice looking ponds and less nice ones.