Gather the images you will use. These can photocopies from old newspapers you found on microfilm, or you can download and print images from sites such the Library of Congress's American Memories. The fair-use doctrine allows instructors to print things such images for one-time use in a classroom.
Print out any images you downloaded. Some of these you might want to turn into cutouts. For example, if you have a print from a painting of Ben Franklin, you might cut away and discard the background.
Paste the images you have gathered onto your poster board to make your sign. You can do this chronologically as a timeline, but it makes the poster more interesting if the sign is a random collage. Once the glue has dried, pin your sign onto the bulletin board.