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.