Why would you make greeting cards out of cardboard?
Cheap
Creative
Unique & Personal
What supplies do you need?
Box cutter (utility knife)
Half of a cardboard box
Amazon shipping boxes are great to use
Copy paper or light-coloured construction paper
Marker or pen
Stapler or glue
Wrapping paper or textured paper of some sort
How do you do it?
Identify an existing fold in the cardboard that will not be too stiff. For example, the fold between two sides of a cardboard box are less resistant that the fold between the side of a box and the top/bottom flap.
Cut a piece of cardboard that is the size of a greeting card when folded in half along the existing fold.
Note that the cardboard can have old packing tape attached to it or can have strange pre-existing cutouts and folds. Any weirdness adds to the uniqueness of the greeting card.
Use this piece of cardboard as a base upon which to staple or glue other cardboard pieces, wrapping paper, or construction paper. These new pieces should provide contrast with the bland colour of the cardboard base.