|Stuff cigarette boxes for the increase of the firmness with paper.|
Take 3 or 4 boxes for the seat face and the back and glue the together at the long sides.
Cover them with self adhesive foil in desired colour.
On the long side of the seat face, glue on a triangular wood stick, thereby the back will become an easy skew.
Glue on the back.
Cover two separate boxes with foil and paste them as arm rest/support leg laterally at the sofa.
For the upholstery cut of fabric in double, large enough for cover the seat and the back. Sew in between a fleece.