Purchase a small scrapbook or art book. They come in spiral or simply bound versions. These types of books are nice because they are acid-free; the paper will not harm the leaves or deteriorate over time.
Collect different leaves. You can do this on a nature walk or over time. Look for leaves that are not dead and brittle; those will break or disintegrate quickly. Instead, pick the leaves straight from the tree.
Glue the leaves into the book using rubber cement or other glue that is acid-free. You can often find this glue in the scrapbook section of a store. Put one leaf per page. You can group them by color or style, or just put them in any order you wish.
Write the name of the tree underneath the leaf. You can use a pen or marker, or type and print it on your computer. Once you have printed it out, cut it down and glue it under the leaf.
Punch two holes in the bottom of several zip top sandwich bags. The holes should be about three inches apart. You will need one bag for each leaf.
Cut card stock into a square small enough to fit in the sandwich bags. Make sure the card stock does not cover the holes, and you can still zip the bag shut.
Glue one leaf to each piece of card stock. Do not put a leaf on both sides; just use one side.
Put your cards in the order that you want to have them in your book. On the page facing each leaf, write the name of the tree it came from.
Slide the cards into the zip top bags and close them up. Cover the zip top area with clear packing tape so they can't be opened.
Slip a binder ring through each set of holes to keep the pages secure and in place. If you don't have rings, ribbon or string can also be used. The book can be read, played with and even chewed on by toddlers without getting messed up.