With Dominions 6, a number of new features such as moddable planes and other features have been added to the mapmaking process. However, the addition of new features may have dissuaded new users from potentially making some new playing fields to contest the thrones of divinity.
After many hours and many fifths of vodka later, I have been able to understand the rudimentary process of making a map without losing most of your sanity in the process. I will periodically put WARNINGS so you don't lose an immense amount of progress due to stupid errors.
What you need:
1. GIMP or some other program that can convert images to .tgas
2. Notepad++ or some other text editor
3. Dominions 6
4. Vodka (optional)

Your first goal is to create a map. It doesn't matter so much the size along it's somewhat square or rectangular so you can play with it. The only color restriction is absolute white (255,255,255) as it is the color to indicate province centers.
!!!WARNING!!!
WHEN PLACING WHITE DOTS BE ABSOLUTELY CAREFUL OR YOU MIGHT MAKE MULTIPLE PROVINCES RIGHT NEXT TO EACH OTHER.
Now to add provinces, set your color to absolute white (255,255,255) Your second goal is to make a 'boundaries' version of the map you just made. CAREFULY add an individual white pixel in the center of each province. Once your finished with this, it should ideally look like this.
https://files.catbox.moe/2u62vs.jpg
Now for the boundary map. Open up a new layer over the map you just made and set the opacity to half or something easy to read the map you just made. Now simply trace the boundaries of the land masses and borders you made with a thin but distinct line with a dark color such as black. Shade in the land provinces with a bright color and the seas with a slightly less bright color. If there's decorative boundaries that aren't going to be overlayed by Dominions boundaries, simply color those areas in black.
It should ideally look like this.
https://files.catbox.moe/b1h17j.jpg
Now that you have made your maps, we have to drop their .tga files into the Dominions map folder. The link should be something like this:
C:\Users\YourNameHere\AppData\Roaming\Dominions6\maps

Now open up Dominions 6, Go into Game Tools > Map Editor > New Map
Now you should see the .tga file opened up in the map editor
It should look something like this
https://files.catbox.moe/movaar.jpg
If you get warnings that some provinces are too close together, it means you likely messed up drawing it and will have to delete the map and the .tga file in that folder and will have to get rid of some white pixels.
Assuming you didn't though, it is now a good time to add terrain types and add provinces connections. This is going to be pretty tedious so press ctrl+a to open up advanced commands and press ? so you can see the list of hotkeys. Right click to select a province and left click to add a connection. Shift clicking adds different kinds of connections and so on.
Once you're done with all that fun stuff press tab. This brings up the Dominions boundary screen and if you press color all it will most likely look pretty wonky. While still in the Dominions boundary screen, press ctrl+b and select the boundary image .tga for the map you made. After waiting a bit, it will redraw the dominions boundaries to be more or less correct. If there's some adjustments you want to make, simply select the province with a right click, select the brush you want, and left click to paint. Smart brushes will try to conform to boundaries.
Once you are done with that and confirmed all your boundaries are good, press tab again and MAKE SURE that all your terrain types and connections are good. Now press save map.
C O N G R A G U L A T I O N S
You have now made a functional Dominions 6 map!!!!

Now if you want to add another plane, this is a little more complicated. Do THE EXACT SAME steps as I mentioned before, including making a boundary map.
!!!WARNING!!!
DO NOT ADD PROVINCE CONNECTIONS OR TERRAIN TYPES BEFORE ADDING THE PLANE TO THE MAP AS THEY MAY BECOME CORRUPTED
Now, open up the map that which you want to add another plane to. In the lower left corner, press add plane and press the map you wish to add. Once the map has loaded you can cycle between maps using the number keys 1, 2, 3, etc. Once you loaded the new map, press tab and add THE CORRECT boundary map with ctrl+b. Add the respect terrains, boundaries, etc. to your new plane.
Now to connect the new planes together press G while selecting a province and type in a number. That designates a Gateway with that specified number and it will connect any other Gateway with the same numbers. Once you have finished, save the map as normal.
https://files.catbox.moe/8pnk0e.jpg

!! W A R N I NG !!!
BE SURE TO DESELECT DEEP CAVE PLANES IF YOU DON'T WANT THE MAP TO GENERATE AN ADDITIONAL CAVE LAYER. SELECTING FOG OF WAR WILL ALSO ERASE ANY NON-DOMINION BOUNDARY FEATURE SO DESELECT IT IF YOU WANT DECORATIVE MAP BOUNDARIES. DO THIS FOR EVERY PLANE.

C O N G R A G U L A T I O N S
You have made a map with more than one plane added!

Now if you want to upload your map to steam workshop find your map file's folder. It should look like this.

C:\Users\yournamehere\AppData\Roaming\Dominions6\maps\yourmapname

Now two files need to be added before you upload: a 256*256 or 512*512 png file to be your map's banner and a dom6ws.txt text file.
The only thing that needs to be in the dom6ws.txt files is
Visibility = "Public"
You can also add have Visibility = "Friends" or Visibility = "Private" if you just want to have friends or just you to see the map.

Once you add those two files it should look something like this, with .tga and .map files for every plane you have.
https://files.catbox.moe/ya9a6s.jpg

Now go to Dominions 6 -> tools -> upload map to workshop -> select the FIRST map in the sequence of planes.

C O N G R A G U L A T I O N S
You have now added a map to the workshop!!!!

Here's the download link for the map I made! If you want to mess around with the files and figure out more of the mysteries of Dominions map making, unzip it in your map folder and tinker around!
https://files.catbox.moe/bog8si.zip
Here is the published map if you simply want to download it!
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3389855161