Remove the seat cushions from the couch. Examine the couch frame. Feel the frame with your hands, looking for breaks in the wood. Be careful as you could get splinters in your hands from broken beams.
Add support to a couch with broken beams by placing a piece of plywood between the couch and the cushions. Measure the couch space, and cut the plywood large enough to give support but not so large it can be seen poking out along the edges.
Sand the rough edges of the plywood to make them smoother. Apply a coat of paint to both sides of the plywood, or cover it with a fabric, using a stapler, that matches your sofa.
Place the plywood on the couch and cover it with the couch cushions. Arrange the cushions so that the firm sides face up.