If you already have a Birchbox subscription of your own and you have received a Gift Subscription you just need to redeem your Gift onto your account. You can redeem your Gift Subscription here by entering your Gift Code - this can be found in your Gift Email.
Once your Gift Subscription is redeemed this will pause your personal subscription and you will not be billed for your monthly boxes until the gift duration is complete. You will still receive a Welcome Box regardless of when your Gift was redeemed. Once the final box on your Gift Subscription is received your personal subscription will resume.