HELP!!!

I need some help! I really really want to go to a 3 column instead of this 2 I have. I can do it if I have really simple directions. I cant seem to find any SIMPLE,PLAIN directions and its a little scary messing with the blog no matter how many times you have backed it up! If anyone knows how to do this or knows someone that has done this , please let me know. Remember it has to be plain english. Thanks!