Log in to your account using the email address the eGift card was sent to. If you don't have an account yet, simply register with that email address.
Your eGift card balance will appear automatically as Moda Store Credit in your account. Add your items to My Bag, log in at checkout, and choose "Moda Store Credit" as your payment method.
If your order costs less than your balance, the rest stays in your Moda Store Credit for next time. If it costs more, you can pay the difference by card.