CH3 is vertically down at the front carbon, but no group is vertically down at rear carbon. So rotate the rear carbon by 60° clockwise to get OH vertically down or 60° anticlockwise to get H vertically down or 180° clockwise to get Cl vertically down. We will consider all three cases and see all of them leads to the same Fischer projection formula.
Third Fischer is the standard Fischer. Let us convert first and second Fischer to standard Fischer.
First Fischer: Interchange Cl & OH and then interchange OH & H at the top carbon
Second Fischer: Interchange Cl & H and then interchange H & OH
All the Fischer projections are identical.
