When you hear a ringing tone indicating an incoming call, simply double tap the outside of either AirPod to answer it. When you're finished, the action is the same -– just double tap an AirPod to hang up the call. That's all there is to it, though you may also want to enable a setting to have your AirPods announce who is calling. Instructions are available here.
If you're new to AirPods, considering buying a pair, or just want to pick up some new tips.