Hi :
To me, the words 'easy' and 'waterfall' are mutually exclusive. Being kinda new to ponding, I'm finding 'easy' and most things in to do with this hobby are also oxymorons. *snick*
To be serious, I can tell you what I did - I bought one of those plastic pre-formed thingies you can find at home improvement stores. (I think they are meant to form cascades or rivulets) Next I piled up some of the remains from digging the pond to a height of perhaps two feet with the plastic pond(?) sunk in at the top. I fed this with the return flow from a power filter, landscaped and rocked around it to hide the ugly plastic, and allowed the outflow to flow over a shaped pile of large rock. Not too bad looking after the wife got the plantings in. Actually we like the way it came out!
I know it's nowhere near as fancy as many I've seen, but that's one attraction of ponding - there's always next year! The best features are - not too expensive, relatively 'easy', and very unlikely to leak. Many fancy falls I've seen have the unwanted problems of 'wicking' the water through the rocks or as water leaves the lip of the falls. I was able to solve that by concocting a kind of bib under the chin of the outlet. I cut it from extra pond liner so it blends well, and sealed it to the plastic with silastic. Now any capillary action at the edge hits the bib and goes right into the pond.
That's my first effort at a waterfall. I'm sure that others have some more worthy ideas for you.
Total : $40 for the plastic thing and a solid day of tinkering, which I enjoy anyways. Best of luck.
Regards,