Use a toothpick to put lots of lines in the waterfall. Run lots of vertical creases so it looks like water flowing. Make some spares in case something happens, make a few extra ones. Once it is dry you can peel it off the wax paper. The caulk is a bit flimsy and it won't hold up well so just cut a strip out of a 2 liter bottle. Make the strip about the width of your desired waterfall but make the lenght several inches longer. You will trim the length when ready to install it.
Hand made in Dorset. This is the bug box (or insect hotel) I was commissioned to make for display at the National Archives at Kew. You can see the stages in the building of this 5' x 3' beast at my website. www.carlscreaturecomforts.com/