Measure the length and width of the sofa cushion using a measuring tape.
Cut two pieces of 2-inch thick foam infill sheeting using scissors to the same measurements as the cushion.
Open up the seam along one side of the sofa cushion using a seam ripper.
Slide one of the foam infill pieces into the cushion, positioning the foam into the top of the cushion and lining up the edges.
Slide the other foam infill piece into the cushion, positioning the foam into the bottom of the cushion and lining up the edges. A cushion that is older and in need or reinforcement will be flat, causing the fabric material on the outside to become loose. Adding the extra 4 inches of infill will plump up the cushion again to its original appearance.
Pin the edges of the cut seam back together using straight pins, then sew the opening closed again using a sewing needle and nylon thread. Trim off any excess thread with scissors before placing the cushion back onto the sofa.