1. First, understand that LayerIndex 1 and 2 refer to the background and foreground images, or the pattern and symbol, respectively. The line immediately underneath it is Pattern Name, and includes the path to your banner file. You can add variations for either a pattern or symbol, or both, it seems.
   
   
2. Open your mod folder and place the alternate/additional files you'd like to include into their respective folders. Back out and go to "Data" and edit the Banners.sbc in your favourite program (Notepad ++ works great, as reccomended by the devs).
   
   
3. Copy the entirety of the line under <LayerIndex>#</LayerIndex> and create a new line underneath the line you copied. This is line 9 for LayerIndex 1 and line 17 for LayerIndex 2 from the stock .sbc config.
   
   
   
   
4. Paste the line you copied on the fresh, blank line. They should line up, spacing is important. If creating a new line messes with formatting, press Ctrl+Z, then TAB on the new line to align it to paste it in the right place.
   
   
   
   
5. Change the new line to path to and describe the new alternate, the .png of which you've already placed in the same folder as the others in its category. Ensure all items are defined and named uniquely.
   
   
   
   
6. Save the .sbc and ensure it's working in-game, then upload. When re-uploading you don't lose pictures or descriptions, so you can always add more later on.

Just stick whatever image you want as the thumbnail into the mod folders main directory and call it thumb.jpg.

The world you use to test the mod will have a screenshot of what you were looking at when you saved and exited, if you did so, and it'll be named thumb.jpg already.

Take this:


and make it look like this:

If you're completely lost as to what any of this means, check out the Official Banner Modding Guide and come back if you're uploading lots of similar or iterated items.

Say you wanted a nice, solid outline of whatever shape for your banner, but also might like to see how it looks with that shape in profile, outlined, vignetted, or in varying stages of decay. Uploading all of those files will take you as much time creating the .sbc and folders as consolidating the uploads with this method, and if there is a common theme, keeping them all together can make small changes easy to put across the board for those of us less intense about our workshop content.