Ning Geng -
Here's the link to a videoI posted recently of a seed cleaner that should be easily adapted for winnowing. If you visit the web site there's a dimensioned drawing you can use to build one.
One thing to note is that the chamber the cracked bean falls down has to be fairly narrow, as does the slit that the air pulls through. So, you want to make sure that seams are as close to air tight as you can. On a recent project I did, I had a lot of scrap plexiglass on hand and so I used nothing but that. It's a little hard to cut cleanly, but there are places to go where you can get the pieces cut for you if you're not comfortable doing it on your own.
The reason I used plexi is that it's food safe. In addition to the glue I used metal tape (that I got from a greenhouse store) to completely seal seams so no air got out. I can't overstate the importance of sealing the seams.
What I like about this design is the triangular blocks that introduce turbulence into the airflow coming from the bottom opening. This turbulence is one key to good results.
Another key is classification. If you pieces of greatly varying sizes, it's difficult to optimize the airflow. You either end up getting a lot piece you don't want falling out the bottom or you remove too much, reducing your overall yield. By pre-classifying (there is a link in the comments to the video for some ideas) you can set an optimized airflow for each fraction.
clay - http://www.thechocolatelife.com/clay/