Seeing as there are no tutorials on here like this, I thought I'd make one.

NOTE:
This works better in older versions of paint.net, because the brush tool got some changes, but the images here where made with the new version.

STEP 1:
Draw an outline for your tile with the pencil tool in the coulor of your choice. It doesn't matter if it looks bad as long as you have the shape.


STEP 2:
Make a new layer over your sketch, and draw over the outline with the brush tool, and then remove the lower layer.


STEP 3:
Now fill in the coulor using the brush tool. Simple stuff.


STEP 4:
Shade in the bigger parts of your tile. I usually use about 5 coulors (2 dark, 2 light and the main coulor) on each section of the tile, notice how paint.net blends them automatically.


STEP 5:
Shade in the details, again I use 5 coulors.


TOTAL TIME:
About 2 minutes

Now hey, that looks alright! with a bit more time it could be a lot better, though, like for example this SMB2 veggie I made only with the brush tool.