Cardboard is a cheap, readily available material that can make a good base for making a suit of armour. No specialised skills or tools are needed to work it into shape. Cardboard is light and capable of holding its shape without reinforcement. For more exotic armour designs, cardboard can be a better material than leather or metal because it is less likely to bend under its own weight for larger unsupported pieces.
Make a pattern for the armour using poster board using an image or pattern for reference. Try to use straight edges or very slight curves in the pattern rather than bending the pattern into round shapes.
Adjust the pattern pieces so they fit together and are the correct size. The pieces should fit together as though they were the armour being made.
Trace the pattern onto the cardboard and cut it out. Mark any places that need hook-and-loop fasteners (such as Velcro) or strapping to be worn.
Cut a second piece of cardboard with the lines perpendicular for larger, flat pieces that need reinforcement. Use the contact adhesive to attach the two pieces.
Attach the hook-and-loop fastener or cut two small slits to feed the gutter strapping to create a strap.
Hot glue or duct tape the pieces together into the final suit of armour.
Use contact adhesive to attach any surface material. This material could be the final surface design of the armour, or it could be to change the surface texture for painting so the armour does not appear to be made from cardboard.
Paint the armour, if needed. Paint that has texture can be painted directly to the cardboard. If the paint doesn't have texture, the armour probably should have surface material to hide the cardboard texture.
Check the instructions of the contact adhesive before use. Most types need to be left to sit for a couple minutes before usage. The lacing in samurai armour can be achieved using shoelaces to replicate the pattern. Cardboard usually fares poorly for making rounded pieces. Cutting such pieces into strips and attaching them to create the roundness will work in some cases.