How to crochet a ripple stitch lap blanket?

Does anyone know of a very easy to follow ripple stitch crochet pattern for a lap blanket/afghan/throw? Not a full size blanket and not a baby one. Just something to throw over while watching TV. How many chain stitches should I start with and what to do next? Maybe about 4 ft wide? I am not sure how to calculate how many chains I need for that width or how many ripples I need. Thanks!