Be specific on the obligations of both sides. His job is to provide you the information/job you specify. There should be a time element and, of course, payment procedure, hourly, daily, percentage, etc. An arbitration clause may be needed. If you have done diligence in selecting a forester, you already know what you need in the contract.
Every job is different, even if nothing more than a different client.
Good conversation with save a lot problems.