Insert the foam and/or other materials to make the cushion fluffy. Once the materials have been placed inside the cushions to your desired fluffiness, then you will use a needle and thread to tack the seam on the cushions.
(Note: you may sew in a zipper or leave one side open such that it may be sealed with Velcro. This will allow for periodic washing of the cushion covers, which may extend the life of the cushions.)