Prior to assembling materials to build the weir, give some thought to the end-product waterfall you want to build. As with any water feature, location is paramount--the more vantage points from which the waterfall can be seen, the better.
With regard to balance, experts recommend adopting a "1/3-1/3-1/3" principle when designing waterfalls: devoting one third of the water feature to rock, one third to vegetation and one third to the actual water. In terms of practical consideration, bear in mind that you'll need access to an electrical source to power the pump.
Knowing exactly what type of waterfall you envision building will help immeasurably in obtaining the end result you want.
The central items to build a homemade waterfall weir include a pump, tubing and materials to build a water reservoir. All materials should be available from home and garden centers or aquatic specialty stores.
Material options for the water course include preformed liners, flexible liners, concrete, stone or clay. Liners are easiest to install but will inflate the price of your waterfall project; concrete, stone and clay require more effort but also offer more flexibility in terms of shape and depth. Regardless of the material you choose, it must form a watertight receptacle to catch the water as it falls.
Building the Weir
After the water receptacle is built, place the submersible waterfall and filter pump into the receptacle and connect the plug end to an outlet. (Be certain the pump is equipped with a ground fault interrupter [GFI] safety switch.) This will function as the weir; place the weir on a large "spillway stone" tipped slightly forward. Attach the hose to the weir using an adapter.
Attach the flexible tubing to the other end of the pump, where water will be removed from the receptacle. Attach the tubing so it is as unobtrusive as possible, behind or inside the objects or rocks the waterfall will be cascading over. The less noticeable the piping, the more realistic the waterfall. Secure the tubing and be sure the water receptacle is securely in place.