Unzip the cushion and remove the existing foam or foam and fiberfill. Separate the foam from the fiberfill. If the fiberfill is not very matted and can be re-fluffed, it can be reused.
Measure the dimensions of the leather cushion cover. You need length, width and height so you can purchase the correct size piece of foam. If there is no fiberfill in your cushions, you will need a piece of foam that is 1 inch larger than your measurements. That way the leather will fit snugly. If you do have fiberfill, use a piece of foam that is at least 1 inch smaller than your dimensions so you have space for the fiberfill.
Buy as many pieces of foam as you have seat cushions. Often, the place where you buy the foam can cut it to size for you. They also usually sell new fiberfill if you need that, as well.
Place the new foam in the old covers. It may be a tight fit and it helps to put on the cover as if you were putting on tight socks or stockings.
Fill in around the foam with fiberfill if it is being used. Be sure to evenly distribute the fiberfill. If it is too difficult to reach the far end of the cushion, use the handle of a wooden spoon to push it into place. Zip up the cushion and replace on the sofa. Repeat with other cushions, if necessary.