How to rotate a chiral carbon
Imagine viewing it from right side:  


H is upwards and CH3CH2 is downwards.
OH is leftwards and CH3 rightwards.
H and CH3CH2 are backwards(from right) so they must be on vertical line of Fischer projection and similarly for OH and CH3.


Alternatively:


Put the dotted wedge CH3 in any vertical position let it be bottom.
Then imagine all other groups to be planar, we see in clockwise order:


CH3CH2 , H , OH

Draw them similarly on the Fischer projection in clockwise sense.!



Note that both are (S)