How do your rescue a drowning victim?

As a trained lifeguard, the first thing I'd say is NEVER go in the water after them. Double drownings happen when an untrained person makes contact with a panicky victim and they both go down. Call for help, if they're close to shore extend your arm or something the extends your reach, take a boat to them if possible, throw them something that floats. If you MUST go in after the person, take something that floats and have them grab on. As the last resort, wait until the person is exhausted and won't fight you, then go in after them. BUT, only is you're a good swimmer. It's not nice to say, but if it comes down to your life or theirs, pick yours. Take a lifesaving cousre. It'll open your eyes.

To rescue a drowning victim it depends on if they are considered a distressed swimmer, active drowner(barely able to keep their head above water and flailing their arms about), or an unconscious drowning victim(sinking to the bottom).
For an unconscious or active drowner you would place one arm over their one shoulder and holding under their arm on the opposite side making sure to keep their head above water at all times. The other arm is used to help you swim back to shore or to the side of the pool. You also want to make sure that they head is slightly tilted back allowing for the airway to be completely open. If they are heavier than you then you can use both arms and grasp under their shoulders approaching from the back, using your legs to swim back. This technique is good but is a little harder if you are not a great swimmer.
If they are just a distressed swimmer you could use a reach pole, ring buoy, or anything you have around long enough like a beach towel or clothing to help in the rescue if they are close enough to you.