To give R/S notation in Fischer projections the same rules are followed as for a molecule drawn in space.
1. Priorities by atomic number are given to the substituents that start from the asymmetric carbon.
2. It rotates starting with the priority group (a) towards (b) and (c). If the group (d) is in the vertical, the rotation in the direction of the needles gives notation R and in the opposite direction to the needles S.
When the group (d) is in the horizontal it is the opposite.

R/S notation to a Fischer projection with two chiral centers.

The R/S notation can be used to draw a molecule in the Fischer projection, or to go from the Fischer projection to the spatial shape of the molecule.