Step 1:
I printed a red plaid texture on white a4 papers
to make the suitcases.
Prefold over this line, then turn the paper.

The suitcase is closed like this...         Done!

Step 8:
Valley-fold over these 2 lines.

Step 6:
Fold over these 2 lines.
Then turn the model.

Step 13:
Press the 2 triangular flaps a little flat.
Then mountain-fold them to the backside.

Step 12:
Fold the edges upwards over these lines.

Step 11:
Fold over these 3 lines.

Step 10:
Prefold over these 4 lines.
Then, turn the model.

Step 9:
Valley-fold over these 2 lines.

Step 5:
Press the spots indicated by the pink arrows
flat.

Step 7:
Valley-fold over these 2 lines.

Step 3:
Fold over these 2 lines.

Step 2:
Valley-fold over these 2 lines.

Step 4:
Valley-fold over these 2 lines. The corners
indicated by the arrows are pulled upwards
when you make these folds.

Step 15:
Pull the corner indicated by the pink arrow to
the left by making an inside reverse fold
over these 3 lines. The edge on the right is
pulled upwards while you make the folds.

Do the same on the other 3 corners, see
red arrows.

Step 14:
This is the backside of the model, the
triangular flaps are folded to the backside.

This funny video also has instructions for the suitcase:

Step 16:
Fix the 4 flaps with tape or glue
(if necessary), see pink arrows.
You can also fix them by folding
in half.

Step 17:
Fold over these 3 lines.