Every website seems to have a different process for redeeming your online coupons. I have seen some sites make you wait until the page you are making your payment on before you can enter and apply a coupon. Others allow you to enter it right up front and make it obvious where the coupon can be entered. Personally, as I do most my shopping online, I prefer to enter the code at when I see all the items in my shopping cart. It just seems to make sense that you would use the code when you are reviewing your shopping cart much like you would present a coupon at checkout while at a brick and mortar store.
So here is the process with step by step instructions on how to apply the godaddy coupon at checkout.
Step 1, the shopping cart will resemble the image below. You will see that there is one domain in the cart at a price of $11.99 and with the ICANN fees it is $12.17.

Step 2, on the right hand side of the shopping cart you will see the order summary. The summary has the order total and a blue underlined text that reads Enter Promo or Source Code. The words are blue and underlined because they are hyperlinked which means you can click them. Once you click them you will be able to see the box to enter the code.
Step 3, after clicking the hyperlinked text you will see an empty box appear. This is the box where you can enter the code. You can enter any code you like and can try multiple codes as well.
Step 4, as you can see from the image to the right, you simply enter the code. Once you have entered a code you then click the hyperlinked Apply to see the total change.
You can try another code, however only one code will be applied at a time. By trying various codes, depending on what you have in your cart, you can determine which provides the best discount. In this case we entered a domain coupon.
Step 5, the image to the right shows the final price of a godaddy domain using the code FAN3 (from the previous image) and clicking the apply link. The domain price is $7.49 which is 35% off the original price of $11.99. With the ICANN fees the total is $12.17. The ICANN fees apply regardless of which domain registrar you use.
Note About Coupons
You can use the coupons as often as you like. You can also send them to a friend or post for others to use. The number or redemptions is limitless.