[ursulan]

I don't really understand how this makes sense.  I know in fischer projections the ones on the left and right are wedges coming at you, but when you look from the top of the molecule, the substituents on C3 and C5 make sense, but C2 and C4 are pointing away...so you just switch them?! I guess I'm just looking for a better answer than that!

[plankk]

Notice that in your picture the substituents on C2 and C4 are coming under sheet. In Fisher projections they have to come at you, so you must imagine that they turn. Or you can draw these molecules upside-down.