Set your filter box so that the waterfall opening empties out where you can see the flow from your viewing area. Giving it a slight turn to the right or left will give the finished stream some character. Keep in mind - every time the water falls over a rock ledge, you want to be able to see it. It's not so important to be able to see it
Stand back and look at the filter box. Check and make sure the lip is facing an appealing direction.
RISE vs. RUN: You will need to set your filter box up high enough (or in some cases, down low enough) for the water to have adequate room to fall over the rocks without running over the edges of the stream bed. Of course, if you want a straight sheet of water off the filter box and into the pond, this isn't a problem, but if you want a more natural looking stream in addition to the falling water, you will need to consider the Rise vs. Run factor. For this, you need to take into consideration the thickness of the rocks you are using in the stream, the number of turns your waterfall will make, and the distance of the filter box from the pond edge. One way to figure this out is to count the number of turns your proposed waterfall will make. You will want to have enough Rise at each turn for the water to have adequate room to fall. Assuming your average RUN is 3' - 5' of stream bed, a good rule of thumb is 8" of RISE for every turn. If the filter box you purchased is 26" from the bottom of the box to the lip of the waterfall opening, and your stream has three turns, you will need a minimum of 24" of RISE for the 9' to 15' of RUN, so you can set your filter box on grade. If you are setting your filter box further away from the pond edge, you will want to put an additional fall in the stream bed even if you don't have a turn there. In this case, you will need to elevate your filter box.
ALWAYS set your filter box on a flat, compacted surface. NEVER set your filter box on excavated dirt. The dirt will settle several inches in the first year alone, and your filter box will settle too, destroying your waterfall and sending all the water in your pond out into your yard. I simply can't stress this enough. Excavated dirt can settle several inches over the course of a year, especially if something heavy, like a 3' x 3' box of water is sitting on top of it. You will notice I use a lot of cinder block under and around a filter box. The reason I do this is because cinder block doesn't settle, so long as you set it on a compacted surface. Also, cinder block is 8" tall when laid on it's side, so it makes it the perfect hight for most waterfall applications.