They were made for eachother. I posted in another thread that I feel like a "bionic man" with both of them on... it's entirely effortless to listen to music, see and reply to texts, take phone calls, etc.
When you are listening to music on your iPhone through your AirPods bring up the "Now Playing" app on your Watch (should be in the Dock on your Watch... if it's not, you can add it from the Apple Watch App on your phone). That will let you control the music and use the digital crown to control the volume of your AirPods.
If you receive a call while your AirPods are in your ears and connected to your phone the Watch will show a little green "AirPod" icon instead of the normal "answer call" button. Pressing it will answer the call on your AirPods.
If you have Apple Music you can load one playlist worth of music directly on the Watch. I won't go through how to do that here... there are millions of tutorials online. However: as previously mentioned, the Watch does need to be docked to sync the music... AND be prepared to wait. It takes a REALLY long time. I recommend doing it before going to bed.
Once you have music on your Watch you can listen to it without your phone around. Swipe up on your Watch face to bring up the Control Panel... scroll to the bottom to find the "Airplay" button (looks like a wifi symbol with a triangle in it). Touch it and it will show you where the Watch can send music... your AirPods should be listed. Select them.
Then go to the Music app on your Watch. Scroll UP to see the "music source" selection. Make sure your Watch is selected... now you should be able to play music from the playlist you synced.
