Remove all of the cushions from the couch and set them out of the way. Measure the seat area of the couch from front to back and then from side to side. Write down the measurements.
Lay a piece of 1/4-inch plywood on a work table and mark the measurement of the couch onto it using the tape measure. Connect each of the lines using a straightedge so you have the outline of the seat. If the couch is long, you may need to divide the length measurement in two and use two pieces of plywood.
Put on safety glasses and cut along each pencil line on the plywood using any standard saw.
Place the properly sized pieces of plywood on the couch seat and replace the cushions.
Examine the back cushions of the couch to see if they also look slouchy. If so, locate the zippered opening, typically on the bottom or back of the covering. Open the cushions and add additional foam or polyester filling material until they are firm. Close the zippers and place the cushions back onto the couch.