Arlene:: hopefully this will help
Buy a foam mattress (egg crate foam, memory foam, your choice) and either cut or fold it into shape. When I did mine, I cut the foam into the same size as the cushions, unzipped the covers and slid them in - this way is more difficult and you have to make sure your covers are in decent shape or the seams could bust. If you don't want to put the foam inside the cushions, you can simply put a piece underneath them. (If this is the case, then obviously you don't need to trim the foam into the smaller pieces)
Mattress size depends on your couch size and how thick you want the padding to be. For instance, some loveseats are about the same width as a double bed. Once you have the mattress, fold it and lay it under the cushions. Super easy and cheap!
For starters you can add a board under the cushions. Then open the cushion, some have zippers and add polyester filling. You can buy bags of it at places like Walmart and JoAnn fabrics. If no zipper...open a seam, stuff and restitch the opening.
I did this project and wrote a tutorial AND made a video! Check it out https://www.charlestoncrafted.com/stuff-sofa-cushions/
There often is a zipper to give you access. You can either add more fiberfill if it's stuffing. If there is a foam cushion, you can replace it, or wrap it in quilt batting to form it up. If there isn't a zipper, you could open a seam that is hidden and then whip stitch it closed with upholstery thread and a sturdy needle.
You might try this https://www.hometalk.com/19912528/quick-and-easy-fix-for-sagging-sofa-cushions
I actually used an old mattress topper (the dog had started to shred it). Cut pieces to length and stuffed into the individual cushions. Made a huge difference and used something that we were going to throw away.
Open the side of the couch and add the stuffing. here is a video https://www.apartmenttherapy.com/sofa-solutions-how-to-plump-up-an-old-saggy-sofa-for-under-30-217134
I redid my clients couch recently, I ordered a single large medium density memory foam, 5" thick to fit the cushion covers. I cut the foam to the width and length of each cover, and wrapped in 1/2" dacron which I used spray adhesive to stick it to the foam. I washed the covers, and only dried to damp, then I folded each cushion to get it in - this is easier when the fabric is damp, then it dries snuggly to the foam.
The back cushions were still good, I just had to do the 4 18" loose cushions which only needed wrapping in dacron to plump them up.
Ty for your info will get the foam and start my stuffing. Appreciate your reply again ty.
If your cushions are fibre filled and you have easy access to the cushions via the zip, you can simply add extra stuffing until you are satisfied. A more long term solution for replacing fibre cushions is to buy replacement sofa cushion foam instead. This will help to prevent it from becoming flattened.