Custom field help

I have all ready created a custom field during the checkout process called “How did you find us” by following the form woocommerce provided. The question I have is how do I make the content of that field, auto fill for a returning customer?
In other words a returning customer has to fill out this field every time they try to purchase a item. I would like that field to be like the other fields and all ready filled out with there previous info so they don’t have to fill it out every time.
Is there a tutorial that will show me how to do that?
Thank you for your time.
I am currently using the latest version of woocommerce, version 2.6.4