Okay, hopefully I explain this well enough for you to make a custom and start a thread. For each of your lines you'll want to make a custom because that makes the lines private so that only you and the person(s) you're writing with see them.
1. On the left side of your journal when you log in look for the heading that says Friends. Under that heading you'll see option "Edit Custom Groups" - click on that.
2. This opens up a page titled Edit Custom Friends Groups. Click on the gray button that says "New". This brings up a little pop up box where you can write in what you want to title the custom. I usually use the names of the characters in the line, but you can title it whatever you like. Click "Ok" after you type in the title.
3. The title you chose will now be in the box on the left. In the center box titled Not In Group you will see a list of every journal you have friended. You must friend a journal to make a custom. (To friend a journal click on the + sign of a persons journal on their info page)
Click on the persons journal name you want to make a custom for then click on the 2 arrows pointing to the right. This will move that journal name to the box on the right titled In Group</i>
4. Scroll down to the bottom of the page and click on "Save Changes". Congrats, you've just made a custom. :)
Hope that made sense.
Now to post/start a thread
1. Look to the left side of your journal again and click on My Diagnosis.
2. On the page that comes up click on the blue pencil near the top of the page. This brings up the page you write/post on.
3. *IMPORTANT* Before you post your thread you need to choose how it's posted (basically public or private or custom).
Scroll to the bottom of the page and look for Show This Entry To: Click on the little black arrow and you'll get a drop down menu with options. Click on Custom</i> then choose the custom you want to save it to.
4. Click Post to kiddg3niu5 and that's it! :)
I know it's a lot of info, but it's easy once you get the hang of it. Let me know if you have questions.