When I did me initial waterfall, I read loads of material and decided to jump right in.
I used a piece of cork for the wall, but if you hunt around you may find a piece of wood that is either good enough,then coloured with say wood dye or upgraded with moulding material.
I tried it on a cork tile thinking I could fit that later, BUT the cork tile curved as the plastic dried and shrunk…eeeek!
so onto the layout and a couple of plaster cast moulded stones, if the moulds are expensive crinkle tin foil and full out then drop plaster in that works really well.
see the curve!
Reference always good, I wanted sandstone type so the cork was a good colour choice,that said by the time I finished the cork had all but disappeared so I could have gone cheaper.
on layout, at this point choose the water colour
decide on the flow of the water
and decide if you want splash back,and how much.
I added blue and white paint as I went for depth and air which is ca-tired in the water, fyi you can over egg this aspect,so less is more.