Continue laying the 4 walls, until you've reached the height you desire. Once you're done encasing the pot with the 4 walls, you need to place 2 longer stones across the top (either front-to-back or left-to-right) to span the walls. Pull up the tubing to gain more length, if necessary, and gently sandwich the tubing in between these 2 longer rocks to hold it in place.
Begin trying to position your first spillway rock on top of your shelf rock. It should jut out over the pond even further than does the shelf rock. Ideally, the tip would line up over the middle of the pond, although this is difficult to achieve. Elevate the first spillway rock in the back, to achieve better water run-off. You can elevate this or any rock in the wall by using shims in small flat stones.
Bend the end of the tubing down towards the pond and place one or more capstones over it. It is under here that the waterfall's "spout" will rest, so to speak. By "capstone" I mean a stone that will partially hide the tubing and/or gently press it down against the second spillway rock. Make sure most of the capstone's weight rests on the rocks between which the tubing is sandwiched or on shims, so that the tubing doesn't become flattened. You'll have to play with the level of the spout, as you begin to fit in the second spillway rock.
Begin trying to position your second spillway rock on top of your first spillway rock. Again, elevate the rock in the back using a shim, to achieve a steeper pitch. One way to think of the placement of the 2 spillway rocks is that they're like 2 shingles on a roof. They're both on a slant, and the top one overlaps the bottom one, forming a continuous chute down which the water can pour.
The position of the end of the tubing that forms the spout can now be determined more precisely, as you size it up on the surface of the second spillway rock. Again, pull to lengthen or shorten your tubing, as necessary.
You're ready to fill the pond with water, plug in the pump's cord, and test the flow of your natural rock waterfall. No doubt, you'll have to make several adjustments before you get everything right. The objective is to get the water to fall as close as possible to the middle of the pond, so that you can minimize water-loss from the splashing that will incur.
There is some compromise involved with your cascade design: greater height equals greater visual impact, but greater height also equals greater water-loss as the splashes will be more violent. Another consideration on height: keep your natural rock waterfall in proportion with the pond. A general rule of thumb would be, the smaller the pond, the shorter the rock waterfall.