Un-used force cards go to the side. Used ones go under the card its on like equipment. I like to offset mine to the left and up a bit so you can see it peeking out.
i position my comited units on the force cards the same way...i just have them laying out infront of my opposite my opponents when the game begins instead of keepign them to the side and attaching them to commit.
For instance Ill deploy vader to my "play area" below my force cards and then move him up to one of the cards to commit.
This way all 6 cards are right next to eachother giving you a very quick glance at the force strength of each comitted unit.
doesn't change anything pratical...just what me and the people i play with have been doing