Once you've got your cassette tapes ready to up-cycle all you need in addition is:
a sharp knife
You'll notice the edges on the side of the tapes have bumps or 'guides' for where the cassette tape rested in the player. You'll need to remove them from one side of each tape so that the base of the cassettes can be glued flush to the cardboard. Take your sharp knife and scrap these bumps off. Be sure to scrap away from yourself.
Line the cassette tapes up and glue the edges of the 4 cassette walls together.
Now measure how much cardboard you need for the bottom. Do this by placing the cassette structure on cardboard and tracing around it.
Cut out your cardboard base and glue it to the cassette structure.
Voila, you now have a cassette tape pencil holder!