Your gift card will be available to activate 90 days after your purchase, as long as you've maintained the policy you purchased.
Once that time has passed, simply log into your Club+ account (or register here) to view and activate your gift card. You’ll find it at the top of your Gift Card page in the “your wallet” section.
Be quick - you only have 6 months to complete your gift card activation.
For more details read this support article with a step-by-step process on how to activate your gift card: