Woocommerce Custom Product

Hi, I’ve been using woocommerce for a few day now, and I absolutely love it! However, I need a way to add a field on some of my products so that customers can input custom text (think custom T-shirt Text). I need a simple and free way to do this. I refuse to pay money for an extension that does something this simple though. Can anyone direct me to an extension that can do this or walk me through the steps to code this?

Thanks!!