Mandy,
Not sure if you are asking how to physically do it, but if so you push down on the handle on top of your sand filter (with the pump off), rotate the handle till it is pointing at the Waste notch, and then turn pump back on. This will suck the water/junk/algae out of the pool and into your yard or wherever the water goes when you backflush the filter. All you are doing is bypassing the filter, so the water is going from your pool, through the pump, then out the discharge. If you are trying to clear your pool and have to replace water anyway to lower CYA, this would be the preferred method (imho) since you are sucking the worst stuff off the bottom and not putting it into the filter to then have to be flushed out of there.
Glenn
Doh, slow typer and Richard beat me to it lol.