One way to ensure that all of the volunteers know what to do before they arrive, especially for large complex events, is to use floor plans. Ask your venue manager if they can provide you with floor plans of the event space. Use the floor plans to visualize how the event will flow, where to place your volunteers and how many volunteers you'll need. Determine the activities that will occur in different areas of the venue and how the volunteers will carry out these activities.
Floor plans allow volunteers to arrive at an event with a clear idea of where to go and what to do requiring far less active management during the event. Floor plan also convey a sense of professionalism to the volunteers which will translate into their interactions with event attendees who will come away impressed by the volunteers, the event and the organization.
You can also use venue maps to develop a staffing plan, detailing how long you'll need the volunteers during the event. If the event is long, consider dividing the volunteers into shifts to prevent fatigue. To be included in the volunteer staffing plan, write detailed job descriptions for the volunteers so they know exactly what's expected before, during and after the event.
Now that you have a floor plan, staffing plan and job descriptions it is time to recruit some volunteers. As you recruit, keep in mind that not everyone will show up. In past event where I've worked with volunteers, about 30% of them would not show up. So I always recruit a few extra volunteers to ensure the event is adequately staffed. Be sure to include some overlap between volunteer shifts so the outgoing volunteers can help the incoming volunteers get up to speed, making the shift transitions seamless.
At least 3-4 days before the event, email all the volunteers the event floor plans, staffing plan and job description. I also make sure to exchange cell numbers with volunteers, so we can text each other during the event if something unexpected happens.