If you don't happen to have an old box of slide mounts lying around, look for some in a thrift store. Each 12" x 14" poster consists of 36 slide holders. You'll simply cut a large picture into 36 tiny rectangles, insert each into a slide mount, and then glue them on cardboard in the same order.
Begin by looking through your old wall calendars for good poster candidates. Not only are calendars typically a standard size, but they usually have gorgeous photos you hate to throw away when the year is over. I like flowers, especially if they have a large central focal point. But you can experiment... even use a real photo if you want.
Step 1: Gather your tools and materials for the project.
1. Piece of cardboard (I'm cutting the sides off this old box).
4. Permanent marker.
6. Exacto blade/box cutter.
7. Slide mounts.
8. Calendar page.