Gather up all the magazines you've got; fashion, music, outdoors... anything you like, and start clipping!
Once you've cut out tons of scraps, start laying them out on your poster board. It's up to you: overlap, rip paper, glue backgrounds first... and once you think you've got it right, glue everything in place.
And you're done! Hang it, give it as a present or make more for your friends! Easy, fun and personalized. ;)