To sign magazine, take you non-dominant hand extended in front of you, palm facing your body, as if it were a magazine. Then run your dominant hand with thumb and finger extended along the bottom edge of the magazine, thumb on the inside and pointer finger on the outside.
Once your baby gets more advanced, you can expand beyond the basic sign book, to include the more descriptive word magazine.
With a common object like magazine, try and use the real object when it is available instead of the flash card. When teaching magazine, it is useful to contrast a book to a magazine.