...
Hit the reply button to open the reply dialogue box
Type your message and press the arrow to send
Your message now appears in the thread!
Collapsing Long threads
...
Expand a collapsed thread by pressing the button pictured below.
Collapse an expanded thread by using the collapse all button.
Notes
For now, threading is only available in channels, and there's no way to turn it off. If threading really isn't your style, you can always stick to using the normal compose box at the bottom of the app.
...