I have replaced the pond liner in the main pond, and it is indeed a lot of work. Looking for leaks is not too hard, but patching them is. After the pond liner has been in use for a while it tends to stiffen up a bit, and conforms to the shape of the pond. Imaging handling something not as stiff as a truck bed liner, but sort of that idea. Anyway, the patching material needs to go over VERY well cleaned material, and it is hard to clean old pond liner that well. I found laying it over the hood of my truck in the sun helps to soften it so it is easier to work.
I use a razor knife to cut it. You may need several razors, they need to be absolutely sharp, and if you scrape against a stone that is the end of that razor. Pond liner is very easy to cut as long as the razor is new.
I put old carpet under mine, but when I install them professionally I use the proper material supplied by the manufacturer. It is sort of like a very thick felted weed mat. This material is harder to cut, but scissors work pretty well, and the sharp (new) razor knife will do the job.