Ask the new owner to sign up for a free Domain Manager Account. They can do so by clicking Manager in the top menu of our site followed by Sign UP.
Once they have created their account, they can provide you with the First Name, Last Name and Username of the account. You may then log into your account and click on the domain you wish to move followed by clicking on the Domain Push option in the service menu of the subsequent page. Use the information you obtained from the new user and complete the fields then submit.
Please note that the domain push option does not automatically update contacts or nameservers, the new user would have to log into their account to make the necessary updates.