To secure a co-supervisor for your Ph.D., involve your main supervisor in the process. As a student, your role is to identify a potential co-supervisor who can collaborate with your main supervisor. Seek approval from your main supervisor before proceeding, keeping them informed throughout. If approved, send an introductory email to the potential co-supervisor, copying your main supervisor. Allow the professors to discuss the collaboration, acknowledging potential administrative work, especially if the co-supervisor is from a different university.