Paste your sample image into the background of your new Paint.NET file. Select an image that fills the entire background. You can download a sample image from the Internet, or take your own with a digital camera.
If you're making skin on Paint.NET, you may wish to apply it as a texture over your own drawing or another image. If so, consider the distance from the camera source. For instance, if you want to draw a close-up face, use a skin texture that is roughly the same distance from the camera.
Create a new layer by selecting "Add New Layer" in the "Layers" window. Double-click on the new layer and label it "Mask Layer," as you will apply white paint to this layer to exclude the background from the composition. This layer should always remain at full opacity.
Apply a third layer, labeled "Drawing Layer," over the first two. Draw the outline of the shape you wish to texture with skin, using the "Pencil" or "Paintbrush" tool.
Alternatively, paste an image onto this layer and use the "Erase" tool to remove extraneous details. You may need to change the opacity of this layer to reveal the skin texture in the background. Double-click on the layer to reveal the "Layer Properties" window. Use the sliding bar to change the opacity.
Return to the "Mask Layer" and use the "Paintbrush" tool, adjusted to a wide brush width, to paint over all areas of the background you wish to exclude. Use the outline in the "Drawing Layer" as a guide.
Create additional layers, labeled as "Shading Layer 1," "Shading Layer 2" and so on, and apply thin, translucent shadows to build subtle gradients. Reduce the opacity to "10" or less for each successive layer. Add as many layers as you need to create subtle transitions in shade and color. Use black paint, or a dark complement, to build shadows. For instance, if the skin texture in your background has a rosy, pink hue, use a very dark green to build shadows. Complementary colors appear on opposite sides of the color wheel; for example, yellow is violet's complement.
Save your file as a Paint.NET (.pdn) file and as a standard graphics file, such as JPEG (.jpeg) or PNG (.png). Saving your file in .pdn format will preserve the layers.