think of a fischer projection like a stick figure wearing a bowtie. The bowtie are the horizontal lines that come out at you. Thus, I believe that that the substituents off the main chain facing outward on the sawhorse will make the horizontal on the fischer.
Its a bit hard to explain, but if you gave an image I am sure I can help you better.
I also found this: It seems to describe what I am saying a little better.
1) The molecule of any chiral compound containing the chiral carbon is viewed in such a way that the two groups or bonds attached to this carbon are horizontal and point towards the observer while the other two are vertical and point away from the observer.
2) the various atoms or groups attached to the chiral carbon are tehn projected on the plane of the paper in form of a cross. by convention the chiral carbon is not shown but it is believed to lie at the intersection of horizontal and vertical line.
3) If the chiral molecule contains two or more chiral carbons, each carbon must be viewed individually in the same manner described above.
4) as far as possible the vertical part of the projection formula thus obtained should represent the longest chain of carbon atoms with the most oxidised carbon ( -COOH in lactic acid, -CHO in glyceraldehyde, -CH2OH in 2-methyl-1-butanol) at the top. This carbon is then numbered according to IUPAC nomenclature.
To derive a three dimentional structure of a molecule from its Fischer projection , we simply reverse the process described above.
