Carefully open the furniture cover to determine what kind of stuffing is inside.
Purchase additional stuffing in the material you need from a home decorating store or a shop specializing in furniture stuffing.
Add stuffing to one cushion of your furniture item until you achieve the firmness you desire. Alternatively, add stuffing to a beanbag chair or other one-cushioned items until the item is fully stuffed. Use a large funnel or piece of rolled-up firm paper to ease addition of polystyrene beads, which can otherwise be slippery and difficult to manage. Add fluffy, fibrous stuffing by hand, ensuring an even distribution throughout the cushion.
Repeat, adding stuffing to the remaining cushions, until all cushions are uniform-looking and equally dense.
Close the cushions. Evaluate whether additional stuffing is needed and add it if necessary.