If you want the LJ cut text to be something else other than the default "Read More", you click the scissors icon in the editor, and write the text that you want to appear.
If you already have a cut, you can change the HTML.
Without cut text, the default looks like this:
But if you want it to say something else, like "Ramona's Girlfriend # 1", you replace that HTML with this:
Text here It'll look like this:
Text here