Firstly sign in to your account, using the forgotten password link of you don't know it.
If you have store credit in your account it will appear as available option in the 'Payment Information' section when you're in the checkout. Simply tick the box to use the store credit, click continue and the amount will be deducted from the total cost. See below for an example of where this is in the checkout stage. Store credit is valid for 12 months from the date of awarding.
Please note, unfortunately store credit cannot be used when purchasing a flower subscription.