I think what you mean is an LJ cut, and you can find LJ's official FAQ on that here.

First, check which editor you use to post your entries. On the update page, do you see buttons that let you bold or italicise text, insert links, and so on? If so, you're using the Rich Text editor. Then, follow the instructions on the FAQ.

(Inserting an LJ cut works the same on a post to a community as it does on a post to your own journal - when you post or edit the entry, highlight the text you want to put behind the cut, then click the LJ-cut button.)

I hope that helps?