User guidelines for citizen registration and appointment for vaccination: https://bit.ly/3e7Papl
For registration or login for vaccination: https://selfregistration.cowin.gov.in/
A person age 45 to 59 yrs with comorbidities wanting to register need to have this certificate certified by Registered Medical Practioner with his or her Medical Council Registration no.
Even Private Practioner fulfilling the criteria can issue the same.
Certificate format to Certificate to identify individuals with co-morbidities: https://bit.ly/3e5nvp4