Unzip the zipper on the side of your cushion. If your cushion does not have a zipper you can cut along the side of the cushion with scissors to create an opening.
Take a length of batting and fold it so it matches the dimensions of your cushion.
Stuff the batting into the cushion and press firmly down on the cushion to even out the internal padding.
Check the cushion. If there are still areas where it sags, repeat the steps.
Zip up the cushion when done or use a needle and thread to sew the hole that you cut in the cushion.