Run a tape measure against the length and width of the seat of your sofa. Write down these measurements. You need to select a cushion top that more or less fits these requirements.
Choose a cushion top that is no more than 2 inches thick. More than 2 inches will make you feel super-elevated when you sit on the couch, making the whole experience feel uncomfortable. Cushion tops are often sold as outdoor-furniture chair pads or long cushions for outdoor benches or chaises.
Pick a cushion that has a color similar to that of your couch so that it doesn't stick out like a sore thumb. Select a fabric that's easy to clean, such as vinyl, so you can easily wipe up stains or vacuum up pet hair.
Vacuum your couch cushions before placing the cushion top on them. Fluff and rotate your cushions weekly so they don't get squashed.