I maintain a thread on the Visual Arts section of a writing site where I post "mini" tutorials for backgrounds for sigs and such, but many of them turned out to be longer than expected, so I'm going to post them one by one. Please let me know if you find any errors (I tend to mess up on tutorials) or if I need to clarify something.
1.) This tutorial is based on the following one, except you don't have to worry about selecting just the right area to make the brush from or anything: newbielink:http://www.gimptalk.com/forum/topic/Creating-grungestract-brushes-4156-1.html [nonactive]
Create a new image with a white background. I usually do mine at 550x200, 500x150, or 350x125.
2.) Open the brushes dialog. Hover over the buttons at the bottom, and click the one that says New Brush. Name it spikey brush 1. Shape: diamond. Radius: 75.0. Spikes: 7. Hardness: 0.50. Spacing: 200. Save. Make another brush named spikey brush 2. Use the same settings, but have the radius at 50.0. Make another new brush named spikey brush 3. Shape: diamond. Radius: 90. Spikes: 7. Hardness: 0.95. Aspect ratio: 9. Angle: 90. Spacing: 200. Make another new one named spikey brush 4. Same settings except the following. Radius: 50. Spikes: 8. Hardness: 0.90. Make one last brush named spikey brush 5. Shape: diamond. Radius: 25. Spikes: 8. Hardness: 0.75. Aspect ratio: 9.0. Angle: 90. Spacing: 200.
3.) Get the paintbrush tool out. Check the box to use color from gradient. Use default gradient (black to white). Length: 250. Get brush 4 out and paint on a new transparent layer until you have it covered. Do the same on a layer above that with brush 3. Get out the eraser tool. Select brush 4 and click a few times in random spots on the top layer. Get out brush 5 and do the same with it. Merge down.
4.) Create a new transparent layer. Paint with brush 1 until you fill it up. Make a new layer and fill it up with brush 2. Move this layer below the other one. Use brushes 1 and 2 to erase random parts like you did on step 3d. Make sure hard edge is unchecked on the eraser tool first. Merge down.
5.) Go to Layers > Colors > Levels, and move the left arrow on the output levels to 30, and the right one to 225. Hit OK.
6.) Filters > Map > Displace on the smooth layer. Set it to 15.00 with the sharp layer as the displacement layer. Filters > Map > Warp on the displaced layer you just worked on. Make sure in the settings that it selects the sharp layer on every option where you need to select one. You may chose to repeat the warp.
7.) Here you have two different options. 7 is one, and 8 is the other. Fill the bottom layer with the color you want the banner to be. Delete the sharpened layer. Duplicate your top layer. Set the top one to hard light and the middle one to overlay. Chose whether or not you want to lower the opacity of the top one.
8.) Add alpha channel to the bottom layer (Layer > Transparency > Add Alpha Channel). If the option isn't there, don't worry about it then. Move the sharp spikey layer to the bottom. Fill the blank layer (probably named Background) with the color of your choice. Set it to hard light. Go to the bottom layer. Filters > Map > Warp, and warp it on itself.