Decide which topic you will cover in your poster board book. The topic can be broad, such as biology or chemistry, or it can be more specific, such as cells or climate change. The complexity of the topic depends on grade level as well as the teacher's expectations.
Gather as many pieces of poster board as necessary (probably six to 10 pieces). These will be the book's pages. Decide what part of the topic each piece of poster board will cover. For example, if the topic is cells, each page can be a different component of the cell (membrane, cytoskeleton, etc.). Use one page for the book's cover.
Make each piece of poster board into one of the book's pages by labeling it, drawing illustrations, and adding captions explaining them. The pages can read like a story or be individual posters that work together within the book. Leave a few inches of room on the left side of each piece, where they will be bound with tape. Use markers and whatever other art supplies you have to make the pages colorful.
Line up the posters and use packing tape to tape them together on the left side. The tape should hold them together enough to open the book and display it to the class.