As is convention in building garden pools, you end up with a load of soil. This is what I did with mine...
The rather large pile of soil
I then covered it with green granite stones of various sizes which will eventually be planted with small ferns.
After much effort, the mountain begins to take shape
The mountain nearly completed
The gaps between the larger stones were in-filled with 16mm green granite chippings and edged with stones. The iris pond can be seen at the back and the exit pipe for the stream is at the lower right of the picture.