When you author your post, you will see an array of icons below the post editor. One of these is the CW icon. Tap on that, and it will split your post in two:

The top bar? The post title. This appears in the timeline.
The bottom part? The post contents. This only appears when folks tap on "SHOW MORE".

Note: Hashtags DO NOT WORK AS INTENDED if they are included in the post title of a CONTENT WARNING post. Add them to content for best results. #CoSoTips