1. Think about what the badge might look like in the end, consider colors and any symbols or words that might be on it. Make sure that what you decide on matches what the badge is awarded for.
2. Open GIMP and create an image 120x37 pixels (This is the official size, it may not be changed)
3. Choose a gradient to use for the sides, in vanilla GIMP there are a variety to choose from, but if you can not find one that suits your needs there are many that you can download online.
4. Select the gradient and click on the top of the image, drag a straight line down to the bottom.
5. Select the middle of the badge, make sure both of the sides have exactly the same amount of pixels.
6. Choose another gradient and repeat step 4
(Steps 7-9 are optional, they are for adding a ribbed texture which can often make a ribbon much more attractive)
7. Save the attached texture and insert it into a new layer.
8. At the top of the layer window select "Overlay" and adjust the opacity to a point that you think looks good, this texture doesn't work great with all colors so don't feel that you need to use it.
9. Merge the two layers
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10. Save your image at this point if you're going to add any further tiers, it will serve as a base. If you do not need to add any icons or words you are done.
11. Copy and paste icons/words into a new layer. (For tiered icons it's good to have them saved in a separate file for convenience)
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Now you're done, if you have any questions ask me or any other artists.

Of course you can alter this method just about any way you want, as long as the image is 120x37

Aye, Well done.