If you have available store credit, it can be applied during checkout on the Payment Method step.
At the top of the page, just above the payment options, you'll see an Apply $X.XX Credit button displaying your available store credit balance. Click this button to apply your credit to your order total before submitting your order.
If your store credit doesn't cover the full order total, you can pay the remaining balance using your preferred payment method.
Your store credit never expires, so you can use it whenever you're ready.