I spent a good portion of the day writing out this article and making sure I spelled things right and capitalized stuff, so yeah, here it is.

First things first, get familiar with the tools at your disposal. I use photoshop, so I'm going to be displaying the photoshop version of the tools, I'm sure your editors if you aren't using photoshop have somethings similar.

The first tools I'll describe are the marquee tool and the wand tool.
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these tools are essential for keeping your images from being blurry, they are used for selecting areas and also can be used to finely delete pixels reguardless of color and if used right can work faster than the eraser tool.

The next are the pencil and the brush.
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know the difference. Pencil is used for solid pixels, while the brush fades out towards the outside of the brush. Both of these are essential to this tutorial.

The last one is the bucket
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use this one to fill an area with a certain color.

There is also a few settings that you must know about for the magic wand and the bucket, the first is the tolerance. for the first part of this tutorial set it to 0, so it selects the color, and only the color that you click on. The next setting is contiguous. You can select it on or off. I cant tell you exactly what it means but when it is on, it only selects the colors in the area, while when it is off, it selects every pixel with that exact color and transparancy in the image.

Well, so to start off I'm going to use a tree for this tutorial, so with your pencil tool, set the size to 1, and draw yourself the wooden part of the tree, close off every shape, then fill it with white. Create a new layer and draw in the leaves, and fill it the same as the trunk. I usually tend to make kind of a half circle and make it all spiney on the bottom, kind of like this. Feel free to draw it however you want to, that isn't the purpose of this tutorial, only a quick way to shade in your drawings.
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Now, using your bucket, fill in the leaves with a green (or whatever color leaves your tree has) and the trunk brown(same as the other).
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Black outlines are horrible, so take a darker shade of the previous colors and with the contigous rule off, fill in the outlines.
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Make sure at this point you finalize any little details in the shape of the tree.
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From this point on you do not need to have both of them to be in separate layers so you can flatten them (or for you edit freaks, keep them in separate layers).

We are now going to start shading our tree, so using the magic wand, select the brown colors in your image.
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break out your brush and in the settings, turn the size down 1, or at most 5. Turn the opacity down to 15% and the flow down to 25%. Change the color to black.
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Having your brown colors selected, go ahead and start drawing around the edge of your tree. the technique I usually use are quick little clicks while flicking the mouse around the edge of the tree.
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Go ahead and darken the area below the leaves, try to keep the shape the same as the leaves themselves.
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Darken in the details of the tree, the knot if you have one, should be very dark. for the rest of the lines, darken the side that you feel is furthest away from the front of the tree.
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Knock the flow of your brush down to about 15% and change the color your brush is using to white. Start shading in the area that isn't shaded black.
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Now that the trunk is finished, select the green colors of your image. Change the color to black and shade in the outside of the leaves, and the underside of the details in the same way you did the trunk.
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You guessed it, chance the color back to white and start shading in the rest of the tree that isn't darkend.
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And there you have it! I'm sorry if I explained any of this poorly, but this is the way I've been doing my graphics currently. The most important thing is practice, if you don't practice you can't get any better. Also expiriment and question. Instead of just taking my word for it, mess around with things and get to know what works for your, question why it does what it does and how it actually works. If you take the time to fully understand the aspect of something, it might take you a few more minutes at first, but it will save you a ton of time in the long run. The other thing is talk to people. If you see someone who does something awesome, ask them how they did it. If they're cool they'll fill you in, if not what's the worst they can do? Tell you no or ignore you.

Let me know what you guys think of this tutorial and if you have any further tips feel free to post them below.