though, i need to remind all of you, this is like, textbook way of drawing these characters. and when i said textbook, it's not ODA-STYLE way of drawing, but more of emphasizing on certain features that you can observe directly from the character. for example, how the distance between usopp's mouth and nose is a bit greater than other characters.
if you just follow these directions, then all you're doing is like being a manga artist from the franchised "how to draw manga" book. you need to add your own twist, your own style to it to make your drawing more lively and unique.
i fail at that. all i do is copying oda's style, and even that i still fail. -__-
that's why i like the stick figure drawing better than my other "good" drawings, because at least it's my own style and i'm not just trying to imitate oda.
anyway, regarding usopp, i'm sure people like cosmic/taboo/chang has their own way of drawing usopp, and the points that they pay attention to might be different from what i pay attention to. the best way is to get a reference picture from the manga and try to observe as much as you can and find your own few important points that can make your drawing look like the way you want it to look like.