That's easy. Find the read more icon when you want to post on the posting page toolbar (it looks like a paper cut by broken lines in the middle). Put your cursor on where you want the article to be broken, then click the icon, presto!
depends, you can create artificial read more with permalink on it with nice button, and then cut your article by excerpt, or you can create a button right after the excerpt where it ends.
another method is when your in html view just insert <!--more--> Code (markup): on where you want your post to be separated