You want to make sure that your markings are large enough to be seen by not only the cyclists, but also any support vehicles that are following the route. You should put a marker a little before a turn, at the turn and a third after the turn so that everyone knows that they have gone the right way. Drive the route a couple of weeks before to look for cars parking on the shoulder that might force you to move your markings into the lane, lest they be parked on and hidden from view. Then drive it again to mark it a few days beforehand. Use spray chalk if at all possible, but it only works well if the ground is COMPLETELY DRY. Once it is down, it will usually survive some rainfall, but a full day of dounpours will wash some of it away. If you use paint, there is striping paint available that shoots straight out of the overturned can, and sprays a tighter line than regular spray paint. Use a dayglo color, and get more than you think you will need, cause if you run out, the store you bought it from will ALWAYS be out of that color when you go back. As for symbols, some sort of arrowhead works well. You could use a capital A (for Alabama), then turn it 90 degrees to indicate a turn, or an A with a circle around it and an arrow on the circle in the right direction. A carat (^) is good, as is an upside down T. The club I ride with uses various of these forms, along with initials for the name of each recurring ride if there are common roads invloved (for instance, the Bikes and Breakfast ride is an arrow with BNB). The 3 day tour I ride each year is called the Autumn Escape Bike Trek, and they generally use a capital A pointing in the direction of the turn. Don't forget to put a straight mark every so often if you are on one road for a long period of time, just so people don't think they missed a turn if they havent seen a marker in a while. You may also want to mark it by bike, just so you can check your work from the right perspective.
Good luck!!!