To add your personalization, there are two ways. First, when your product is added to cart, you will see this screen
Below is the desktop view, circled is the "add a note to order", this screen pops up on mobile as well in a condensed view, but the notes spot is in the same area.
Alternatively, you can add order notes directly in your cart before you reach the checkout process. Click to view your cart and you will see a screen like this, your personalization notes can be entered into the personalization box shown beneath the product image and information