How do I create smooth curved lines in the vector costume editor? I need it to make a character smile in a project, I'm quite new to snap after all. 
Also, I tried using free draw but it wasn't smooth.
Your best bet is probably to just use a different vector editor, like the scratch costume editor or inkscape. The devs know that the snap costume editor is very limited, and not very good (believe me, most snap users who create costumes use some external program).
Thank you.