2. Select the bottom layer. Using the magic wand tool, select all white. Paint inside the lines! The top layer is the line-art.
3. If you have a really dark color, select the top layer (line-art) and just invert it.
4. File - Save As - BlahBlah.xcf to save as a GIMP !ONLY! file. To export it for photobucket/internet use; File - Save As - BlahBlah.png. Click the second option. Then click all boxes. Make sure quality is at 9. Save.