<!-- more --> where you want the break.
UPDATE: Well, if you edit posts using plain text/HTML, put
<!-- more --> where you want the break. If you edit posts using the rich text editor, which is the Tumblr default, click this toolbar button to insert the break:
If this helps you, please Like this post.
"Read More" breaks in Tumblr Text posts are not a new feature, but I have only recently added support to BS. Text posts with HTML enabled can mimic every other post type and therefore are the most flexible. I recommend using HTML. Otherwise the results can be unexpected. Illustrating this would be a laborious and unappreciated exercise, so trust me. If your content matters, consistency matters. The break should be done like this:
<p>This paragraph is above the "Read More" break.</p> <!-- more --> <p>This paragraph is behind the "Read More" break.</p>
BS has an Appearance option to change the break link’s text. If you use your Tumblr blog to sell crafts or something, for example, “Purchasing info” may make more sense. Then the more detailed purchasing information can be hidden from your index pages, after the read more break.
Do whatever you want. The power is in your hands. Just read more.