Write the word the way it sounds. If, in your story, a character yells, "Stop!" and you want to indicate it being drawn out, write it like it would sound out loud: "Stoooop!" It's doubtful someone would shout that word and emphasize something other than the "o": "Stopppp!" isn't realistic.
Concentrate on vowels. Most words that get dragged out concentrate on a vowel: "Stooop," "Nooo," "Soooo" for example. There are, however, notable exceptions. J.R.R. Tolkien often used "s" emphasis when Gollum spoke in "The Lord of the Rings." Gollum would always refer to "My Precioussss." Hard consonants, like a "t" or "b," are unlikely candidates for being dragged out.
Keep the additional letters to a minimum. "Noooo" vs. "Noooooooooooo": the first is every bit as effective as the second in telling the reader that this bit of dialogue should be dragged out. There is no need to be excessive when using additional letters to indicate this type of speech.
Exercise restraint when indicating a word being dragged out. Think of using this technique the same way you would use an exclamation point--sparingly. When you do use it, the impact will be that much more effective. Instead, use the text outside the dialogue to indicate the speaker's vocalization: "No," she said, her voice trailing to a whisper.