This hole in the layout is what had to be filled with the valley section.
My first step was to construct a kind of box that would fit into the hole from below.
Into this 'box', I then placed Styrofoam to make the rough shape of the valley sides I wanted.
I could then remove it from the layout and do additional shaping. The wooden skewers are holding the styrofoam pieces in place until the glue has dried.
I could then add plaster
While this was going on, I also made progress on the landscaping such as the weir and lake around the valley, taking advantage of the fact that I could still stand where this valley piece would eventually be.
I could then fit the somewhat completed valley in from below and make a mock-up of the waterfall (using some toilet paper).
When I was happy with the overall feel, I covered all the gaps with plaster.
Painted and added grass and other vegetation
and finally the waterfall!