There is a plus up in the top right group of icons. When you click it, it gives you options to choose from and asks if you want to make a thread, post, link,ect. Once you are in that page there is an option for what magazine you want it to go in. [FYI I have been using kbin for three days so I could be wrong.]
Go to the magazine that you want to create a thread in, hit the + in the top bar, and select "Add new link" or "Add new thread". There's also a select box for the magazine once you hit one of those.
For a microblog post, make sure to hit "Add new post" and again, there is a dropdown select box for the magazine. Also, you can add a hashtag to make the microblog post show up in various magazines.
This is my understanding as well, @a@kbin.social¹. And with this understanding, I concur with @anonymouse:
they made things overly complicated
Indeed. It’s not a meaningful separation particularly when the microblogs have no significant enforcement of being “micro”. The /feeling/ I get is that kbin threads (which are very poorly named as @s6original discovered) are for high effort often lengthy content with a graphic, formatting, and more thought-out content. But kbin supports formatting in both which makes them redundant.
I suspect if someone makes a lot of short off-the-cuff “threads” a moderator might want to have a word with them.