It's actually simple to use the lj-cut tag!
When posting, select any sections you want to have hidden behind a cut. In the case of fanfic posts, this would be the majority of the entry--leaving only a small amount outside the cut (such as the title, author, summary and warnings).
The lj-cut tag is created by using the following code:
*Your text (fic, images, links, etc.) go here.
That's all there is to it!
If you want to change the default 'Read more...' text for something of your own, you only need to add:
|<lj-cut text="your text here">|
The area inside the quotation marks (seen in red) would be the text you want to appear with your cut tag. (This might be a portion of the fic or whatever else you wish...)
Questions? Feel free to contact us!