Remove all the cushions from the couch. Inspect the frame, looking for broken and rotten wood. This is the area you will need to replace.
Examine all of the areas where the cushions are. If the couch's springs or platform are broken, cut a piece of the plywood to fit into the broken area. This new plywood will create a platform to support that area of the couch and prevent sagging.
Continue around the rest of the couch. Saw off the old pieces of wood and replace them with new places where necessary, using a hammer and nails.
Add the foam padding to the cushions that need it. Unzip the area of the cushion you need to refill and add the foam padding. Smooth the padding out once it's inside the cover. Zip the cushion back up.
Put the cushions you removed in Step 1 back where they belong.