OK, prep work is out of the way, it’s time to move on to the structure itself.
Take a look at the rocks you have. The most important rocks are what might be termed the "spillway" rocks. The spillway rocks are the ones directly over which the water will cascade.
The spillway rocks should be relatively flat as opposed to rocks that are more rounded in shape. They should also have sharp, squared edges. Water will cascade more cleanly over such edges. When rocks have blunt, gently-curving edges, some of the water tends to follow that curve and trickle back under the rocks.
The idea behind the selection of spillway rocks for a cascade design is to choose rocks that are most likely to channel the falling water in the precise direction in which you want it to go. How you lay the spillway rocks is also important to this end, as you'll see later.
In addition to seeking out relatively flat rocks with sharp edges, see if you can't find rocks that are slightly cupped. That is, occasionally you'll come across rocks that curl up ever so slightly at the edges, leaving a depression in the middle. The natural channel in such rocks will be greatly advantageous for the creation of the spillways in your cascade design. Their raised edges will help keep the water from deviating where you don't want it namely, behind the rocks.
You'll essentially be building four mini-rock walls around the pot, to box it in. Make a small trench for the tubing to sit in under the rocks, so that the rocks don't weigh it down. This will keep the tubing free, so that you can slide it through the pot up or down, at will. This gives you the leeway that you need, since you won't know at exactly what height you'll want the water spouting out until you've finished laying the rocks.
You may have been wondering what the flower pot in the supply list is for. You’ll need a pot about 11” high with a drainage hole in the bottom that matches the diameter of your tubing. The pot functions as housing for the tubing within the cascading structure for the waterfall. You could easily substitute something else that might work better and can use either a terra cotta or plastic variety. The idea is to have some sort of housing to hold the tubing in place, while you lay up the rocks all around it. This housing won't show when you're finished: it will lie hidden at the center of your rock work.