Main Rod:
1. Cut along for all parts but do not throw away the inner piece of frame (the biggest cardboard part).
2. Remove the inner piece and glue the bonding surface. (Spray glue is recommended)
3. Put the inner piece back to the frame.
4. Paste the frame to the thin frame (190 grams piece) and press all surface with a flat plate. The inner piece keeps the frame shape when assembly.
5. Remove the inner piece.
6. Assemly the Joint Reinforcement pieces (the smaller cardboard pieces)
7. Fillet out the corners with knife.
I avoid to use sand paper to the surface. It makes rough surface and not good for painting.