You can tie them if you would prefer. About every 6 inches take a small stitch through all layers with crochet thread or 6 strands of embroidery floss and tie a square knot. Clip the ends about 1/2 inch long.
If you want to quilt them, I would go with straight lines, perhaps following a seam. For this I like to use a decorative stitch on the machine to make sure the fabric on both sides of the seam is secured.