Step 1. Get 10 digit account number (on top right of the page)
Before giving your 10 digit AdWords Customer ID, perform your due diligence. This includes understanding agency’s past performance and client testimonials. Also, insist on NDA and non-compete agreement to safeguard your data and business specific information.
Step 2 : You will receive an invitation from the agency to join or link your AdWords with their MCC.
Once you have sent your 10 digit ‘Customer ID’ to your marketing agency point of contact, confirm with the agency manager that your account has been linked with their MCC account ?
As soon as the agency has added your account in their MCC, go to your account settings on top right to give access to the certified agency as a ‘Client Manager’
Step 3 : Accept the request and provide the access level as 'Client Manager'
Give access rights to the certified agency as a ‘client manager’ only, this way the agency can make changes to your account to optimize it further.
As a best practice, do not give access your agency as ‘Administrative Owner’. Administrative Owner has full management access to your account, including being able to grant and remove access for all users (which includes you).