Once you have your objectives and an idea of how to measure, consider the constraints.
Consider the time, cost and scope of your project. If any of these variables change, at least one of the others must also change to accommodate this to maintain a quality output.
Planning ahead for your project may not help you deliver exactly what you expected, but it will prepare you for the unexpected.
There are a variety of project planning tools that can help you manage your team of volunteers. Here are some resources: