This is my first instructable so let me know how to improve it.
Step 1: Open up the picture
First, flip the picture over and bend the top a little bit. a "bulge" will appear and you can tuck the exacto knife under to loosen it. Do this on both sides. You don't have to pull the side flaps up all the way down to lift the top one.
Once you have the side flaps up, use the knife again to carefully dig under the top flap to make that rise.
Squeeze the sides to pop the Polaroid open.
Step 2: Choose a digital picture
Step 3: Importing into Photoshop
The next step is to set up guide lines. If you don't want to make your own, download the Forged Polaroid.psd I have included. Otherwise, there are two ways to manually do this. The first way is to drag down from the rulers (ctrl+R or Apple+R to enable the ruler) to the appropriate lines.
The second (more precise?) way is to go to the menu View and click New Guide. You want guides at the following locations:
Once these guides are in place, paste in the image you want to use and resize it (ctrl+T or apple+T). What you want to be visible should fit to the inside of the guide lines, but be sure to overlap to compensate for alignment errors.