All gift management is conducted on your account page. You should see the relevant tab:
When you click this, you'll be given two options to send a gift:
Head to Your Accounts and click Copy register link. Send that link to the person you wish to gift the membership to and they can register their gift. The membership will begin when registration is complete!
Printable Gift Certificate
Head to Your Accounts and click Gift PDF to download it. Print and send (or email) the gift certificate to the person you wish to gift the membership to. Instructions are included for redeeming their gift. The membership will begin once they have completed registration!