tool path question

Ok after playing around with the Vector demo i am starting to get a
grasp on things. I just drew a box and chucked a pen in my mill to
see if I could get it to draw a box and sure enough it did. Small
stuff I know but I was sure amazed! I understand the lines for the
box I drew and converted to G-code was the tool path. What I would
like to try is machining two shoulders approximatley .345 (don't have
the plans in front of me) on a square piece of brass. I guess I will
draw the part the way its supposed to look when its finished and then
draw the tool paths, is this correct? Concering the tool paths will
I have to make a line every .12 (the amound I can usually take in a
pass with the mill) for the tool path?

Thanks for all the help and patience :)
Matt