3 steps to follow:
Measure the height of your sofa backrest. The cushion cover can be larger for a low backrest to ensure comfort. And if the backrest is high enough then let it be at least shorter by 2” of the backrest.
Measure the depth of the sofa seat, this will determine the number of cushions you can layer or not to ensure optimal comfort.
Measure the width of the single seat of the sofa, this will help you determine the width of your cushion covers. Ideally, the cushion should be shorter than the width for the setup to look more aesthetic.