I take a lot of my inspiration from Viria13 on DA, so if you ever want to check her tutorials or videos, you should! But, um, I don't know how to explain it... I'll try though. Let's see, first of all, I use Paint Tool SAI, and there is a type of brush (it can be found for every tool, like the crayon, marker, etc.) called Spread. Play around with the settings, like the blending, density, edge hardness, etc. Also, use several colors in the same family, like for example, different shades of brown or reds. I think that's the best I can explain it, hope it helps some!