Polymod Tutorial
Create a folder in the mods folder and rename it to whatever you want.
Doing this manually is not a problem, but it would be faster if you copy-and-pasted the Template in the mods folder.
The information for the mod is stored in two files. Those two files are _polymod_icon.png and _polymod_meta.json.
In _polymod_meta.json, you can define the mod name, the name of the author, etc.
Now, about _polymod_icon.png, it's just a simple .png icon for the mod. Just keep in mind that whatever image it is, it will always be squished into a 150 by 150 resolution. So a 1:1 aspect ratio is recommended for your image.
When you're done with that, you're basically good to go!
Here's an example mod by Master Eric > here <