I think the UI of this forum is quite user-friendly.
To post a message? There's a large and prominent "Post Topic" button on the homepage.
To reply? Others would post something like "Say something..." at the very bottom of their message.
To change your card? Go to someone else's homepage and you can initiate a card change from the menu next to their name.
—It's okay if I make a mistake. Even if I miscategorize something, the administrators will kindly help me sort out the tags and categories. —I'll get it eventually!
…Moreover, if a tutorial were released, some people might complain that they couldn't find it.
At this point, it's really necessary to have this:
