Examine your sofa cushion to determine why it is baggy. Remove the slipcover and check to see if the foam is still in good condition. Check the seams on the cushion cover and the stretchiness of the fabric to be sure it is not ripped or torn.
Replace the foam in your couch cushion. Buy upholster foam from the fabric or home decorating store. Measure the dimensions of another cushion on your couch that is not baggy.
Trace the outline of the cushion onto the foam, and use a knife to cut the cushion out of the foam. The cushion needs to be the right size to match the other cushions on your couch and to fit properly.
Reinforce the seams on the cushion cover, by sewing over them again with your sewing machine. You may need a stronger needle if the upholstery is heavy.
Stuff the cushion back into the cover by pushing it into a smaller shape, and then releasing it inside of the cushion cover. The more difficult it is to fit into the cover, the tighter the cover will be, which will prevent new cushion from becoming baggy.