In case anyone was wondering the program I use is fire-alpaca. I usually do very clean lined art on it, but recently have been trying my hand at this very sketchy and less controlled style, inspired heavily by Don't Starve, with my own little flair.
It really depends on the brushes and settings you use to get the marker look.
My smoothness (usually set to 40) is turned down to 16 and I use an analog brush to get that charcoal affect for the lines. Go over the outline a bit till I am satisfied with the thickness of it.
After you put your base colors down you can start placing markings for whereabouts you want to shade. Just skitch sketch in the lines loosely till you are happy with their appearance
I use the analog brush even for the marker like shading, use the color tone I want the shading to resemble, then set the layer to multiply and adjust the opacity.
The key to the sketchy still is to honestly not care too much about looking clean and complete. Just allow your hand to doodle freely.