Step #1. Open Safari browser.
Step #2. Go to http://www.youtube.com and select a video you wish to play and tap on play button.Step #3. Now press the Home button, and video will pause automatically.
Step #4. Swipe up from down to bring Control Center.
Step #5. Tap on the Play button, and your video will resume.
If you have the iphone headphones you can skip Step #4 and #5 and just push the play/pause button between the volume controls.