I can't help with Untamed icons, but I'll happily go look for some tutorials for you.

What software do you (intend to) use? I can go look for specific tutorials if you give me a bit of info.

For now, here are some that are for Photoshop (because it's by far the most common, or used to be) :

The first thing you need to know is how to get the aspect ratio right:
https://dtissagirl.livejournal.com/167217.html

Second is lighting and coloring and sharpening:
https://pure-flames.livejournal.com/1908.html (lighting, contrast, coloring)
https://icons-concrit.livejournal.com/9523.html (sharpening)


You can also browse these communities if you are looking for something in particular:

https://icon-tutorial.livejournal.com/
https://good-tutorial.livejournal.com/

Feel free to come back and ask again if those weren't what you were looking for.