OK.. if it's rich text you just use the icon to create a cut. If it's html you'll have to use the html code <...lj-cut text="text you wish to write and close it with (<.../lj-cut>) but take out the three dots in each of those... that was just to show you how to do it.
Rich text is easier technically, however it can be wonky. Some people prefer html b/c it's not nearly as wonky.