When you are writing your post, you will see up in the QuickTags the word MORE…type as much as you want for the intro, click on MORE, then continue typing.
As well, you can add it manually by insterting this: <!--more-->
where you want the break to be. Clicking on the MORE in the QuickTags does this exact thing for you.
Don’t forget the readme.html file, as there is some helpful information in there too