Roundabouts have several exits, you can stand on the traffic island on the exit leading your direction. This way you can be sure that every car is going in your direction.
To choose the best spot, it's a good idea to walk around and see what the speed is like and where cars can stop. Sometimes it's better to stand at one of the entrances, where cars can easily stop, than at the exit where they speed up (even if all of them would be going in your direction). If you're standing at one of the entrances, make sure your sign indicated the direction you need.
See also traffic lights.