Choose a place that has plenty of parking and easy access to roads going the way you want. Access to restrooms would be very desirable and maybe food and drink depending on the time of day.
Investigate all possible routes. Use an online search or other resources to make certain a route is usable and safe. With Google Maps you can turn on the Traffic layer to identify road closures and / or construction.
Plan on roads based on the type of cars in the tour, don't pick Interstates if you have Model A's. Stay away from gravel and the dust it creates.
Note all safety concerns such as potential separation due to traffic lights, stop signs, or congestion, left turns on busy highways, and absence of shoulder.
Note all markers and landmarks: road signs, stop signs, mile posts, gas stations, etc. to help clarify the route.
Note construction zones, steep grades, gravel or fresh paving.
Most people would like a stop after an hour or so, maybe sooner if the roads are rough. A stop with a restroom would be good.
Use common names, what people will see on the route. Use left and right instead of North, South, East, and West.
Try to keep to the speed limit, but base your speed on the quality of the road and type of cars in the tour. Keep enough space between you and the car ahead of you so you can always stop safely.
When on busy roads allow enough space for others to pass safely.
Except for the leader older cars should be up front, newer cars to the rear and modern cars at the end. Older cars will be the slowest, best to keep them up front so they don't get left behind.
The leader should give out their cell phone number with another person in the group as a backup. If there are so many cars that the leader cannot see the last car then designate a “sweeper” that is always at the end of the group and who can inform the leader if there is a problem.
If there is a problem have only the problem car and the sweeper pull off and inform the leader so the group can stop at a safe location.
Find a place where everyone can park together, best if the lead car goes to the nearest point in the parking area so it can be the fist car out.
Decide in advance if the drive back home will be in a caravan or individuals travel at will.