How do you add filler to a leather couch cushion?

https://www.youtube.com/watch?v=CplVJw-x0OM

If it were my couch, I would call in a reputable upholstery professional to take the cushions and have them install zippers for you. That way you can add or replace the foam and/ or batting to plump it back up.

There are professionals who can fix stuff like that but you could open the seam yourself in an area that you can't see and then add stuffing, then sew it back up.

I did this for my couch and it came out great here is the link to couch filler https://amzn.to/2u8Mmn1

Try boosting the support underneath the cushion. If that doesn’t work, you may have to add a zipper to add filling. Add a throw pillow to disguise the issue.

I would do as Adrienne suggested if it were my cushion.

I would use a seam ripper on the cushion back seam to pull out the old foam, and then replace it with a new memory foam cushion. I did this recently for a client's sofa, and got the cushions on Amazon since I live in the boonies. get the 1/2" dacron to wrap the cushion in before inserting. Measure the leather cushion to see how high the cushion had been usually 5-6" and then the width and depth of the cushion to order a new one. Fold the cushion a bit to get it into the old leather cover. Then use carpet thread to sew it shut, a curved needle works best.

I agree with Naomie and Adrienne, a pro will fix this easily and properly especially since it was an expensive piece. In the meantime, you can slip a piece of plywood under it.