A drogue can also steer the boat. I have seen a few different methods of creating a drogue. The November issue of Cruising World had a great sidebar "Rig a Drogue" in the article "Up the Creek without a Rudder". This took the basic drogue design shown below and modified it to allow steerage.
All the needs to be done is to extend the length of the bridle and run each end to blocks located forward of the Genoa winches (usually the strongest winches). You can then use the winches to shorten a side of the bridle and pull the drogue to one side or the other creating more drag and causing the boat to turn. As is usually for sailing control still comes down to a balancing act. You have to experiment with balancing the amount of weight on the drogue with how much sail is up to have control and keep a straight course.
The diagram above shows an actual store bought drogue line but a drogue can be made with materials already on most boats. The drogue line can be your standard anchor line with knots tied in it for drag and a couple sections of anchor rode chain for some weight.
More detailed discussions are available in the links below.