If there is a chance to untighten (I´m sorry, if this is not the correct term) the cushions without any damage, that´s the first step.
Option #1: cover these parts with lead foil. Then you could do some crinkles to the into the lead foil, depending on the wear and tear the seat should have.
Be careful that the parts would fit onto the seat once you put the lead foil on the parts - maybe you need some trimming. Add some filler (i. e. Gunze) if you like and then paint.
Some wash and/or drybrushing would do the rest.
Option #2: put some tissue onto these parts, dip some water with a drop of white glue onto using a brush. I would recommend to put the tissue plain to the parts without or without any huge
crinkles (I haven´t seen any worn seat cushions so far but no rule without an exeption....). When the tissue has dried once again some filler and paint.
Option #3: sand off the paint and some glue onto the parts. Mayabe you have to repeat this a few times to soften the styrene. Then you can do some crinkels using a knife, a little screwdriver
or whatever you like. If you´re satisfied, let it dry. Next step is painting.
Option #3a: Soften the styrene using glue, then press something onto the parts to create some texture, i. e. rough sanding paper.
I would recommend to try these options first, using some spare parts. You will see which one is perfect to your eye.