I did this program at my work yesterday and I did it a few years ago to. You have to make a simple duct tape wallet first. Here is a link to Wikihow for how to make that. Once you have the wallet you move on to the exciting part of this project the monster.
For the monster flat sheets of duct tape work best. Wal-Mart, Michaels and AC Moore all carry these. Measure for approximate size of the monster you want. Flip the duct tape sheet over. Draw your monster on the paper side, pen works best. Cut out your monster, peel off the paper backing and stick it on the wallet. For the city have a black duct tape sheet handy. Measure the length of you wallet and cut strip that length by three blocks high (the blocks are pre-marked on the paper side. Using the paper side to guide you cut down and up to make the outlines of the city building. Then peel of the backing and put it on the bottom of the wallet over the bottom of the monster to make it look like it is terrorizing the city. Use sharpie markers to make details on the monsters.
I did the red one years ago and the green one last week.
I did this rainbow bird monster at the program yesterday. I discovered that my gold sharpie makes some good lights on the buildings.
There you go.