Collect waste paper, old cardboard, or wood pulp
Soak the material in water
Break it down into a pulp
Mix the pulp with water in a large container
Add sizing or adhesive if needed
Pour the mixture onto a flat screen or mold
Spread it evenly into a thin layer
Press out excess water
Dry the sheet completely
Layer multiple sheets if making thicker cardboard
Press the layers together firmly
Trim the edges to the desired size
Cut or store the finished cardboard sheets