You should take along the completed referral letter or an equivalent letter issued by your medical scheme at a vaccination site, as well as your ID and medical scheme details (if available). The referral form /  Annexure A: Covide-19 Vaccination: Request for administration of an additional dose can be found on the following link: 

You can also take the vaccination code for the additional dose if you are able to register online

More information is given in this circular