Alright, so here's a tutorial for creating a signature in GIMP. (Between Easy-Intermediate). You will be making something like this:
1. Open a new document, 420x90 pixels.
2. Have your background layer selected. Go to Filters>Render>Clouds>Difference Clouds. In the window that pops up, raise the detail to about 7. This depends on how detailed you want the signature. Mine looks like this:
3. Now select the brush tool. Make the opacity 60, and begin brushing in black with a grunge brush. Then switch the color to white, and continue brushing with grunge brushes, but not as heavily.
4. Now go to Colors>Invert. This should give you a nice looking background.
5. Duplicate the background layer, and go to Filters> Blur> Gaussian Blur. Make the blur method IIR, and set the horizontal and vertical to 13. Now set the layer mode (Mode>Grain Merge/Multiply) to either multiply or grain merge. This depends on your taste. For me, I chose grain merge.
6. Make a new layer, and title it Render. Get a render you like, and place it into this layer. For my signature, I will use this render:
Planet Renders // Renders: Click image to close this window
7. Take the render layer, and duplicate it. Then go to Filters>Blur>Gaussian Blur, using the same settings as before. Change the opacity to where you think it looks best. I chose to make the opacity to about 60. This depends on your render.
Here's what I have:
8. Now, still on the render duplication, go to Colors> Brightness and Contrast. Fool around with the settings to get a more defined look to the render.
9. Merge down the layers starting from the top so that you have one layer, which should be named background already. Now go to Colors>Color Balance. Play with the settings. Be sure not to distort the color of the render too much, but this effect makes the render blend in with the background.
OPTIONAL: Perhaps your image looks a bit fuzzy, so go to Colors>Brightness-Contrast, and fix that. This makes it even better looking.
10. Now you may add text. A note about text: Only great and experienced signature makers can pull off a fancy looking text, as they are hard to match the signature. You should try to stick to simpler text.
a. For an outlined text, select the A for text. Select your text and an appropriate font, and do so in the color white.
b. With the text layer selected, go to Layer>Transparency>Alpha to Selection.
c. Duplicate this text layer, and then [BOLD]move the copy below the original[/BOLD].
d. Go to Edit>Stroke Selection with the color black as your main color. Set it to either 2 or 3 pixels, and then stroke. Select>None, and you have your bold text. Merge the original text down onto the copy.
OPTIONAL: Add a shadow by going to Filters>Light and Shadow>Xach-Effect. Then move the text layer above the highlight and shadow layers, and have them in the order, from the top: Text, Shadow, Highlight, Background.
Here is what I have:
(To learn how to add color to the white in the bold, PM me.)
11. Now for the border. Borders are simple, yet make a signature a whole lot better. There are two types of borders I use in GIMP, which are the normal single line and the see through triple line. For mine, I'l use the normal single line. Make a new layer, go to Select>All, and then Edit>Stroke Selection. Stroke it with about 8 px in black (or another color you see fit) and you have your border.
And that's all! Now you have a nice signature. Here is the end result:
I hope this helps.