Emphasizing words or expressions through adding letters

Is there a list or web source showing the correct usage to extend words for dialog writing? For example, making so... so-o-o; or ugh..., ughhh? I'm the type who mentally corrects all misspelled words so I don't know how to write intentional "wrong words". I see it used in novels all the time and would like to know if there is a list or do they just make it up? Thanks for any help you can offer! -JSJ