You must mark exempted candidates as 'absent' on the attendance register. You must not put a '0' or any other information except 'absent' or 'A'.
You must give each candidate a written record of the exams from which they are exempt. Give this to them at the same time you give them their statement of entry. You need to give a list of candidates with exemptions to your invigilators for any exams where you have one or more candidates with exemptions. You need to keep a record of your communication with your candidates about their exemptions and the lists you give to your invigilators.
Section 7 of the Exemptions regulations supplement has information about exemptions requested in error and/or candidates taking exams for which they are exempt.