I assume that most people will just open their ftp program and tick the boxes. When the user does this, the question is writable by whom?You know all of the files and folders that are in the normal XF download? They all need to be writable. The specifics of how to do it, whether it is chmod or chown or some other method entirely will be down to you, your host or your server admin to figure out.
Owner of course. Group? World?