Note: With this method you never have to switch groups to see what the stereochemistry is. Using the "drive-the-car" analogy, what we are doing here is defining the steering wheel in relation to the steering column (which is normally behind the steering wheel):
1. Determine the priorities of the four attached groups from highest (1) to lowest (4).
This is the same as in all methods.
2. Draw the steering wheel. Draw a curved arrow around from 1 to 2 to 3 and back to 1 and note which direction this arrow goes, clockwise (cw) or counterclockwise (ccw).
The distinct feature of this method is that you make a full circle, from 1 to 2 to 3 to 1, completely ignoring the lowest priority group.
3a. If the lowest priority group is behind the steering wheel, then this is the standard orientation: clockwise is R and counter-clockwise is S. (Turning the steering wheel clockwise turns the car to the right--R.)
Note that Fischer projections are best depicted as “bow ties” with horizontal groups coming out and vertical groups going back from the central carbon atom.
3b. If the lowest priority group is in front of this curved arrow, then the assignment is reversed: clockwise is S and counterclockwise is R. (Basically, you are looking at the steering wheel from the perspective of the engine compartment!)
Ready to try for yourself? See http://www.stolaf.edu/depts/chemistry/courses/toolkits/247/js/isomers and select "R,S Quiz."