How to apply Green Glue to drywallThis YouTube video is from AcoustiGuard and it demonstrates how to apply Green Glue to drywall. They make an important point: stagger the seams when you add your second layer of drywall (the one with Green Glue on the back of it).
Also, as I've said before, if you can get your hands on some QuietRock 510 to use as your second layer of drywall, even better! Otherwise just use 5/8" sheetrock. But if you're not removing the existing sheetrock that's on your walls, you'll get better results if your second layer is soundproof drywall, with a layer of Green Glue in between.