So what is it with getting a straight answer about the Kirby drive belt?

I have been trying to find out how to replace my Kirby little black belt that drives the brush beater bar on my vaccum. I have searched all through this stuff and no clear answers. Is there a trick to the entire madness? I put the belt on, turn the dial to tighten, put back on then put to green and the thing won't engage. What in the hell am I doing wrong?