Picture-tutorial under the cut ;)
(Kinda huge screen shots, sorry...)
Step 1:
Write whatever you want in your post...
Step 2:
Highlight the text you want inside your LJ-cut
Step 3:
Use the LJ-cut-button. A new window will open and you can write what you want the cut-text to say
Step 4:
This is what you'll have
Step 5:
You can use the HTML too... This is what it'll look like.. Don't touch this if you don't know what you're doing :P