Heavily starching the backing and spray starching the top before layering help a ***lot*** to prevent ripples and puckers. Starch stabilizes the fabric so it doesn't stretch underneath the sewing machine foot. Also, spray basting helps because it keeps all 3 layers in continuous contact with each other (instead of every 3").
I should add that even after layering, spray starching helps. In that case you need to spread out the sandwich on a large flat sheet. Spray starch and let it dry (a fan helps speed this up), then repeat several more times. Do this for both top and backing. It should help.