To vote in a referendum, you need to be eligible to vote and registered on the electoral roll. The requirements for eligibility and registration may vary depending on the country or organization that initiates the referendum process. In general, to be eligible to vote in a referendum, you must be an adult citizen of the country or organization that is holding the referendum. In Australia, for example, you must be an Australian citizen aged 18 years or older and enrolled to vote.

Once you are eligible and registered, you will receive a ballot paper with the proposed alteration to the Constitution or the question to be put to voters. You need to indicate your vote by clearly writing "Yes" or "No" or by marking the appropriate box on the ballot paper.

It is important to note that the rules and procedures for holding a referendum may vary depending on the country or organization that initiates the process. For example, referendums may be initiated by the government or by citizens, and they may be binding or non-binding. Therefore, it is important to check the specific rules and procedures for the referendum you are planning to vote in.