This is something I made for placing RCS thrusters in the right positions in the first try without having to go back and forth between the VAB and the hacked gravity launchpad.
Features:
Display thrust and torque forces caused by RCS or engines.
ΔV readout for monopropellant RCS.
Dry center of mass marker (DCoM).
"Average" center of mass marker (ACoM).
Center of drag marker (CoD) for assisting in parachute placement.
Ability to resize editor's overla
Not exactly easy to understand if you have no background knowlage in math and physic, but wikipedia about couple of forces and about torque is good start.
Basicaly, if you put in consideration rotation only on one axis (either of putch,yaw or roll), you can sum up all of forces that act on your craft to only one force vector on some distance from COM. One for each side of distance from COM, actualy. You need to sum up all other forces into other force vector that act on oposite side from CO