SB and I did this project last week... it was a 3 day project for us.
Day 1 - go for a great leaf walk, carrying bags to collect all our goodies.
Day 2 - sharpen crayons (with my eyeliner pencil since I haven't used it in ages anyway) and iron the crayon shavings and leaves in between wax paper (low iron heat - just enough to melt the crayon)... in between newspaper! Tape the edges and tape onto the windows. This really is pretty when the sun shines!*
Day 3 - get out our Tree book and identify as many leaves as possible. Oak, aspen, maple, etc. Talk about maple syrup, acorns, those helicopter seed pods, etc.
We always made these in the fall when I was little. Lots of people who have seen School Boy's have asked me how we made them. Making them was the easy part - getting the craft area cleaned up of crayon shavings - and my ironing board (well, of course some got spilled) - now that was the tricky part.