The title explains it all: How to Make a Wireless Wii Sensor Bar.
A correction: When I say AA battery in the video, I actually mean AAA battery. Yes, I suck at life, but don't care enough to change it =)
If you have any questions, post on the comments and I or someone else should be able to help you.
-Also, I am planning on making an IR transmitter with a longer range. I noticed that the range on this sensor bar is only about 10 or so feet; it is really finicky or doesn't work at all beyond that. I'll let you know how that goes.