I love this method because I don’t have to use that little slidy-sewing ruler.
Much, much faster and with a nice crisp edge.
(You could also tuck ric-rac or something under before you sew down your fabric.)
I like using grosgrain ribbon because it only requires straight lines and has a nice finished look.
Sew all edges down, just like the fabric.
Now you can make dozens of them and impress all your friends at baby showers with your amazing domestic skills. And remember, don’t stress too much about perfection. Their main purpose after all is to clean-up curdled baby barf. But that baby’s mother will thank you for giving her such a stylish accessory to protect her formerly spotless clothes.