What you need:
- good, solid cardboard
- pressed cardboard
- fabric for inside, outside, bottom and top
- soft filling
- scissors, glue, sewing machine, needle and thread, stapler
Step 1: Cardboard
Cut 9 cardboard squares of the same size. 37 cm is an ideal size.
Step 2: Fabric
Cut the inside and outside fabric for the sides in 1 piece. I measured it by laying the squares next to each other on the fabric. Leave some extra fabric in the ends. (5 cm extra in the ends, 2 cm between the squares)
The bottom fabric should be a bit bigger than the squares. You can better take the same fabric for the bottom as the outside, but I didn't have enough of the pretty fabric, so I took this ugly white fabric.
Step 3: The Sides
Sew the inside and the outside fabric together to make four 'pockets' to put the cardboard sides in, two in each pocket. Make sure the squares fit in thight. Then sew the ends together. Put the squares in to test if they fit. You can already see the box taking form.
Step 4: Bottom
Take the squares out again. Sew the edges of the bottom fabric. Then sew it to the sides on the inside of the box.
Put in the squares and sew the top edges by hand.
Glue some inside fabric on the nineth square and put it in the box.
Step 5: Pressed Cardboard
Cut two squares of the pressed cardboard as big as the box is wide. Glue them together.
Step 6: Soft Filling
Cut the filling as big as the square. Staple it on the square.
Step 7: Top
Cut inside and outside fabric about 16 cm bigger than the square, on each of the four sides. Sew them together to make a 'pocket' in the middle to put in the square with filling. The outside fabric needs to be bigger than the inside fabric to cover the filling.
Don't close it yet. First sew the long edges of the side pieces, you can do that with the sewing machine. I put 1 layer of pressed cardboard in them, but I think you can use regular cardboard as well. The fourth side of the square and the corners of the side pieces have to be sewn by hand.