How to manage a volunteer

Treat them like a consultant

Start the relationship with a contract to clarify your expectations.
Help your volunteer learn how to be a consultant.   Most generalist marketers are employees, with no consulting experience.  They know how to do marketing;  they may not know how to do consulting.  Structuring the relationship will help them adjust to being a volunteer consultant.

Finish the relationship with a close form.  This is the way you “pay” your volunteer.  It contains your results and your volunteer needs this information for their resume, to build their career.   Every marketer needs a resume that contains concrete accomplishments but in a large corporation it is hard to measure the impact that one employee makes.