Edit: Adding some more serious things, mainly details for those keen on following it up and using the techniques as good as possible.
First of all, this tutorial is made in photoshop, and mainly talks about tools available in the same program. The version does not matter, seeing as I stick to the simple options.
If you are using paint tool SAI or a similar program, you will notice you can't find the same brush or most of the shortcuts. I WILL make a tutorial for SAI eventuallym but for now I hope it will at least help you a little bit if I say you can use the regular pen tool on low opacity, paired with the brush tool for softer 'gradients'.
The way of selecting the colours in between and going over your shadows is one pretty much every painter in photoshop uses, and is often seen as one of the most basic elements of painting.
Ofcourse, you can always smoothen things up with the smudge of blur tools, there is nothing wrong with that, but please... use them for touchups and added effects, don't rely on them like I used to when I started out. Looking back at my hours of faffing about was somewhat hilarious.
I hope this tutorial will help you, but ofcourse... if you have questions, feel free to leave a comment or PM me, or check out other tutorials in the groups designed for them ;D
-end of edit-
FINISH'D 8D ohohoho
Hope you like it!
Sorry for the length of this tutorial. It's a huge file, I know.
I wanted to show as much as I could but... well I could use some training on the 'short but sweet'-element.
Eric and Kyle aren't mine, ofcourse. They, and Southpark as a whole, belong to Matt Stone and Trey Parker. I just used them for the tutorial seeing as I had this painting planned for a while.
By the way, just ignore the massive rambling. I love text boxes.