Go on a walk outside and collect leaves in your bag. Make sure to point things out to your kids on the walk - pretty leaves, different colors, etc.
When you get back home, dump the leaves out on a piece of newspaper to examine your stash.
Cut 2 pieces of contact paper that are the same size. We decided to cut our sheets so that they are about an inch smaller than a piece of construction paper, so they would fit nicely in our construction paper cover.
Peel the back off of one piece of contact paper. Arrange your leaves on the contact paper. When the leaves are stuck down, peel the back off the second piece of contact paper and carefully lay it on top of the first piece. This works best if you start on one side and "roll" the second piece over the first to avoid bubbles. Press the two pieces together well.
Repeat step 4 as many times as you need to to get all of your leaves "pressed".
Use two pieces of construction paper as covers to your book. Cut them as needed to fit your contact paper.
Staple the contact paper leaf sheets between the two covers.
Print our leaf template and cut it out. Trace onto construction paper and cut it out. Glue it onto the front cover of your book and write "my leaf book" on it. You could also trace the leaf directly onto your book cover instead.