Wear and tear over the years cause sofa cushions to lose their comfort. The stuffing in cushions gets lumpy and becomes misshapen, ruining the cushions' appearance and function. Try beating the cushion against a hard surface to see if the stuffing breaks up and disperses. Refill the sofa cushions if efforts to redistribute the stuffing don't work.
Measure the length, width and thickness of the sofa cushions, using the tape measure. Search for zippers, buttons or openings for removing the old cushion material. Inspect the material so you'll know what you need. Generally, foam is used for cushions on the sofa and stuffing is used for the pillows.
Remove the old cushion foam from its protecting cover and place it on top of the new memory foam. Cut the new memory foam with a sharp knife. Press down on the new foam to make cutting it to the right size easier.
Stuff the new foam into the sofa cover and zip or button up the cover. Place the cushions on the sofa and sit on them to make sure they're comfortable.
Unzip the covers to open the sofa pillows. Grab a handful of the stuffing and stuff into the pillows. Use a chopstick for pushing the stuffing into the corners. Stuff the pillows until they are at a desirable thickness.
Don't wash pillows or cushions with their stuffing still inside.