After you have chosen your editor (or agreed to edit a product) these sample contracts can be used by the editor or the writer (client) to clarify expectations. Jurisdictions vary, so start with one closest to where the work is being done.
Canadian: Standard Freelance Editorial Agreement (also in French) from EAC
US: Letters of Agreement from EFA
Sample Freelance Contract from Copyediting.com
UK: Terms and Conditions from SfEP