writing a good reference letter (your own)

So I’m getting ready to change jobs and asked a colleague if he would be a reference for me and that I might need a reference letter as well. He said he’d be glad to do so and asked me to write it for him, then he could make a few changes, if necessary, and sign it.
So, I’m a bit stumped, now that I have to write my own reference letter, about where to start. Anybody know any good resources for this kind of thing?

And isn’t that kind of a strange request for him to make?