I in assigning R/S stereochemistyr

[future_vet]

(There is no problem statement etc.)

I am having a very hard time figuring out how to assign R/S stereochemistry on certain compounds. I can't rotate molecules in my head without making mistakes, and so I rotate them on paper.

Let's take this example:
http://avogadro.chem.iastate.edu/CHEM331/331FinalKeyF03.pdf
Go to page 6 please, we have to label compounds.

When a molecule has 2 atoms in the plane, + 1 in the front and 1 in the back, I can rotate them. I keep one atom fixed, usually #2 (4 being the smaller, 1 the largest), and rotate the other 3.

However, and this is where I need help, I don't know what to do when I have molecules such as the ones on the left, where 2 are in the back and 2 in the front, and none in the plane of the paper/screen.

How can I rotate those on paper in order to assign R/S stereochemistry?

Thank you for any help you can give me!

~J.

 

[future_vet]

Actually I found this:
http://chem.pdx.edu/~wamserc/C334F99/4notes.htm

Thanks!