A bit complicated to explain...
Maybe you should check out some tutorials from James Quick (https://www.youtube.com/playlist?list=P ... hi240G3W_T) or other tutorials on Youtube...
Usually it works much better to see what you have to do instead or reading what you have to do.
Good luck!
Damn, still can't post URL's due to my 'reputation' being to low <img src="{SMILIES_PATH}/icon_e_confused.gif" alt=":?" title="Confused" />
Anyway: look for James Quick / Yann Granjon / Scirravideos and many others.