Before you continue, Copy and Paste in back (Command + C then Command + B) all the letters because you will need them later. Hide them for the time being so that they won’t bother you. Now, focus on the first letter and having the original shape selected, go to Effect menu > Sketch and apply the Note Paper effect using the settings shown. Do the same thing for the other letters and when you are done select Expand Appearance from the Object menu.