I agree with previous comments. It's going to be extremely difficult to fit the liner properly without having full access to all four sides. It's difficult to see from the photo just how far the waterfall outlet overhangs the pond so it may be possible to tuck the liner under the lip until you can do the job properly. You'll have to accept that it won't look very good until then.
I’m going to finish emptying it out and have a good look, however I’ve just had a quick look and there is a slight lip so i think it may be possible to slide the liner under slightly. Thanks for your help !
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