Use the wool to stitch 'seeds' onto the two pieces of red fabric.
Pin the green fabric to one piece of red fabric right sides facing and sew in place using your sewing machine or by hand.
Do the same with the other side then sew the top together leaving a small opening.
Pull the watermelon right sides out through the small opening and press.
Stuff with hobbyfil or scraps and sew the opening closed.