This is a pretty open-ended question that does not have a straightforward answer. The short answer is that you will probably want to create some sort of path planning algorithm that converts the words/images into a set of waypoints. Then you will want to create a controller for that robot that hits all of those waypoints. An example controller might be something like:
- Turn so you are facing the next waypoint
- Drive straight the correct distance to the waypoint
- Go back to (1.) and repeat until all waypoints are reached
The tricky part then is accounting for where the marker or pen is on the robot. If it is at the center of rotation, that would make things easier, otherwise you would have to account for it moving while rotating.
