Couch cushions that have gone flat are not very attractive and aren't very comfortable to sit on either. Heavy use, organic debris, and environmental factors all play a role in breaking down the poly-foam padding that rests inside the cushion's upholstery fabric. When a cushion goes flat, the best way to fix it is to completely replace the foam. If you try to replace only part of the foam, the new foam will decay faster than it should.
Remove the cover from the old flat cushion padding by unzipping it, and turn the cover inside out. Lay the cover on a flat surface.
With the tape measure, measure the width of the fabric band--or the height of the couch cushion before buying new poly foam. The band will normally be somewhere around 15 cm thick, but it is best to confirm the width beginning.
Measure across the top of the cushion with the tape, going from front to back as well as from side to side. Add 3 cm to each dimension, because if the foam fits snugly into the cover, both the foam and the cover will last longer.
Mark the dimensions of the cushion's top plus 3 cm each way on the poly foam using the marker.
Plug the electric carving knife into the power source and turn it on.
Hold the knife straight up and down and carve the foam by following the marker lines. It will probably feel a little bit awkward to hold the knife straight up and down, but it will slide through the foam with almost no resistance.
Put your hands inside of the fabric cover. It should still be inside out.
Grasp the corners of the foam through the cover. Begin to work the cover slowly over the foam, turning it right side out as you push it onto the new foam.
Zip the cover so it is closed and begin to gently pinch and tug the corners of the cushion until the new foam fills them in.
If your cushions do not have zips you will need to look for the place the cushion was sewn shut and open it with a seam ripper. When you are done fixing it, you will need to hand-sew the cover shut.