In order to redeem your store credit make sure that you are logged into your petermillar.com account. Once logged in, shop the website as normal and add any items you would like to purchase to your cart. Upon reaching the checkout page, your store credit will automatically be deprecated from the total due. The deduction may be found on the right hand side of the checkout page, where your total is listed.
If you need assistance with logging into your account, follow the steps in this article.