This instructable will show you how easy it is to make a ball, we'll make a 9-ball, but as you can see in the last step anything goes.
Just 5 easy steps for a great WoW effect.

PaintNet-for-beginners.pdf
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Step 1: Things You Need

Offcourse you'll need Paint.Net,
but for the ball effect you also need the Shape 3D plugin.
Download the plugin from the mirror site, the main download links seem to be dead.
Or just download the zip below.

Unpack the zip and place the Shape3D.dll file in ~\Program Files\Paint.Net\Effects folder.

Shape3D_1_2_6_0.zip
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Step 2: Lets Get Started

Start up Paint.Net. You can leave the canvas at its default size (800x600).
With the Rectangle Select selection tool draw a rectangle across the canvas.
About 1/3 in size.
The 9-ball has a yellow ribbon, so choose yellow from the Colors window and with the Paint Bucket tool fill the rectangle.
Now Deselect the selection and add a new layer.

Step 3: The White Spot

On this second layer, with the Ellipse Select tool, draw a circle.
Hold down the shift key while you drag to make a perfect circle.
With the Move Selection tool (the white arrow) you can position the selection in the middle.
Fill the selection with the Paint Bucket tool with white (right-click will use the secondary color).
Again Deselect and add a new layer.

Step 4: Time to Number the Ball

On the third layer we'll add the number.
For this just use the Text tool.
I used Arial, size 84, bold and underlined (sometimes it's underlined sometimes not, I googled ;) )
You'll notice a small cross at the bottom-right of the text, use this to position it better.

when you're satisfied about the result you can flatten the image. This puts everything on one layer.
You'll find it in the menu Image --> Flatten, or use Ctrl+Shift+F.

Step 5: Now for the Really Hard Part

No really, this is so easy you'll wonder if an instructable is even necessary.

If the plugin was correctly installed, you'll have a Shape 3D menu item in Effects-->Render.
Choose this to start the plugin.

In the Texture Map (top-right) choose Plane Map (Scalable).
That's it, instant ball effect.
You can play with the Object Rotation and Lighting settings for a nicer effect

Step 6: Paint.Net Advantage

A plus for Paint.Net is, that it remembers the last settings used.
So it's really easy to make all the other pool balls.

Undo a few steps, to just before you flattened the image, back to 3 layers.
Change the color of the background layer, the number on the third layer and apply the Shape3D effect again. The settings are still the same, so you can make all balls look alike.