For many of us, it's a simple matter of attaching a long vacuum hose to the suction hole under your skimmer basket. Before doing that, fill the hose with water to "prime" it and remove most of the air. Then attache one end to your vacuum head, and the other into that skimmer port hole. The pump should be on high, and the multiport valve on filter (unless you had lots of gunk to vacuum directly to waste). Once you are done vacuuming, remove the vacuum equipment. Then you'll probably need to backwash the filter. If so, backwash for about 1 minute or so, then go to "Rinse" until the site glass gets clear. Once the water in the glass is clear, go back to filter.
Important - Always turn the pump OFF when changing mulitport valve settings. If your pool set-up is different than what I described above, let us know and someone will help.