A functional mod created with this approach:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3391711960

Open your Workshop directory, where you create mods in order to later upload them to Steam Workshop.

The path is:

Create the necessary folder structure with the following folders and files:


Here's the example of workshop.txt file:

In the next section we will describe what goes into the following folder:

We start with the following folder:

At first, make two files in this folder:

Poster.png can be any image used as a poster for in-game mod manager.
Here's an example of mod.info file for in-game mod manager:

Those two files are the poster and the descriptor.

---

At this point we can create the following files that actually contain our mod data:


MyFlier.png is the actual picture you want to use for the flier. You need to know its dimensions (width, height).
myFlierDefinition.lua is the file that adds your flier to the list of other fliers.

Inside of definitions, you need to maintain the same fler ID (name) across all definitions. Let me show you how. The flier definition (id, object name) will be MyNewFlier.


Here's an example content of myFlierDefinition.lua:


Pay attention that we used the same name MyNewFlier twice:
- Once in "newFliers" list. Here we simply create the defnition.
- Once in "MyCoords" list. Here we add coordinates of the rectangle that should be revealed on the map by reading the flier.

You can find the needed coordinates in Debug mode/in beta42 by right-clicking on a world tile and checking coordinates. You want coordinates of North-west and South-east corners of a rectangle.

If you want to add multiple fliers, you will add them to the list like this:
newFliers = {"MyNewFlier1", "MyNewFlier2}

---

Finally, we describe what goes inside of the flier in terms of the picture and the failback text.

This is the example of Print_Media_EN.txt.
Note that we use MyNewFlier as a name again inside of Print_Media_<NAME>_title lines etc.

Make sure to use width and height in th line Print_Media_MyNewFlier_info that are the same as the width and the heightof the picture.

---

At this point the flier is ready to display he image and show us coordinates on map, but you may want to add a "failback" text that you can read by clicking small icon on bottom left of the banner.

To fill this text, write into Print_Text_EN.txt file.
Here's the example


All that is left is to test that it works in the game (spawn in -debug mode multiple fliers until yours shows).

Launch the game and click WORKSHOP in the menu.

Select the option to "Create and update items", and select your mod to be published.

After that you may want to go to Steam Workshop and add extra formatting to Steam Workshop description of your mod.

I would recommend to save your final mod description formatting in a text file, as updating your mod from the game can overwrite your carefully structured description again.