Measure the width, depth and thickness or height of the love seat cushion, using the existing or second cushion as a guide if available. The cushion may be L-shaped, T-shaped or square depending on the style, so be sure to include these dimensions as well.
Cut a piece of foam to these dimensions; the density and compression of the foam you use is up to you. You can cut the foam yourself with a jig saw or other blade, but many fabric and upholstery stores will cut the foam for you.
Cut the fabric to the cushion dimensions, allowing 1/2 to 1 inch on all sides for a seam allowance. You will need one large piece of fabric for the top, front and bottom of the cushion. Three smaller pieces are also needed: one piece for each side of the cushion and one piece for the back. Rip the seams of the old cushion and lay out the fabric pieces as templates if possible.
Pin one of the side pieces to the larger piece of fabric with the right sides facing in. Sew the fabric pieces together along the 1/2 to 1-inch seam. Repeat with the second side.
Pin the top edge of the back piece to the larger piece of fabric and sew together.
Sew a zipper or velcro to the final three sides of the back piece of fabric and to the three sides of the larger fabric piece. This allows you to open the loveseat cushion to remove the foam so the fabric can be washed.
Trim excess material from the seam allowance, making sure not to cut too close to the seams. Iron the seams flat so they don't add unwanted bulk to the finished cushion.
Turn the cushion fabric right side out through the zipper or velcro opening. Sew on piping around the top and bottom edges, if desired, for decoration.
Pull the fabric cover over the piece of cut foam, ensuring the corners are even and flat. Smooth out the fabric and zip or velcro the cushion closed.