You should tell the referee that the candidate is being considered for a position and that they have given their permission to make contact with them for the purpose of an employment reference.
There may be specific points that you would like to clarify from a candidate's CV, cover letter, interview notes or other information provided during the selection process. Only share information with the referee relating to specific claims that you need to verify or clarify.
Tell the referee the information being collected is for the purpose of evaluating the candidate's suitability for the role and that any information provided by the referee is confidential and their comments will not be released to the candidate. If you communicate with the referee by email, please include a statement to this effect.