Sofa cushions use a wide variety of fillers: down, shredded sponge, cut-to-fit sponge, cut-to-fit foam, shredded foam, rubber-coated foam, memory foam, polyester fibrefill, shredded cotton fibre fill, down around foam, polyester batting around foam, and PVC pellets -- otherwise known as styrofoam pellets. With all of those options, don't settle for brick-hard sofa cushions that came from the factory. Revitalising your sofa cushions into customised comfortable cushions is a zip or a few seam pops away. You can make comfortable sofa cushions without the help of professional services.
The easiest method
Examine your cushions for a zip or back seam.
Unzip the cover or use a seam ripper to cut the seams. Insert the seam ripper into the seam and push the "hook" of the ripper until it touches a stitch. Press against the stitch and the "hook" will cut it. Remove the stitching along the back seam. Remove about 25 cm (10 inches) of stitches if your cushion has a soft filler, or enough stitches to remove a foam insert.
Buy polyester fibrefill, cotton fibrefill, shredded foam or shredded sponge. About 2.5 to 5 kg (5 to 10 lb) per cushion is usually enough.
Grab handfuls of fibrefill, foam or sponge. Insert the materials through the cushion opening. If you have a cushion with a soft filler, leave the filler in place and add to or remove the filler for each cushion until it feels comfortable when you sit on it.
Continue to stuff the new filler inside the cushion, periodically sitting on or leaning against the cushion until it feels comfortable.
Zip the cushion closed or resew the back seam with a needle and button or upholstery thread. A tight whipstitch will hold the seam sufficiently.
Remove the cushion covers from the filler insert by unzipping or removing the seams with a seam ripper (see Section 1, Step 2).
Measure each cushion cover. Measure the width, length and gusset width of each cover. When the cover is on the cushion, the gusset width may be thought of as height rather than width. Strips of fabric along the four sides of a cushion that create a "box" effect are called welts or gussets.
Determine what foam or sponge you want to use. Fabric and upholstery shops sell foam and sponge. Online cushion replacement retailers sell foam and sponge in large sizes so you can cut the material to fit several cushions.
Determine the amount of foam or sponge to buy based on your cushion-cover measurements.
Place the old sofa cushion on top of the new foam or sponge. Trace around the old insert with a marker. The tracing will be slightly larger than the old insert, but the new material will eventually be compressed.
Cut just outside of the tracing lines with long-handled scissors or a carpet knife.
Insert the new cushion inserts into the cushion covers. The foam/sponge might be tighter because the material is new, but you don't want the cushion cover seams to strain. Remove the foam/sponge and trim it smaller. Zip or whipstitch the cushion cover closed.