You need a suitably sized cardboard box, more cardboard, padding, cover fabric, contact plastic, two ice cream sticks, tape and glue.
Cut open the Corners of the front wall of the box and fold the front in to the height of the seat, be sure to reserve space for the padding.
Bend the lid in to the backrest and cut off the excess cardboard from the sides. Tape together.
Place the padding on the seat and back, glue it a little with stick glue. Place the upholstery fabric on top of the cushion and secure it by gluing the edges behind the backrest and to the sides of the seat, at the front directly to the sofa frame. Make the armrests from a pile of cardboard or a narrow box, cover them with contact plastic. Also cover the lower part of the sofa frame with contact plastic.
Glue the armrests in place with hot glue and, if desired, ice cream sticks on the armrests.