How do you make wrinkles in GIMP?

I've seen tutorials for Photoshop concerning how to make wrinkles and such, but so far, I've only seen a confusing GIMP tutorial from the manual site concerning how to make wrinkles. I'm pretty familiar with GIMP and the tools, I just do not know how to go about making wrinkles on my shirts and blouses.
Any help would be appreciated!

Same method as in PS. Create a new layer and select a colour a few shades higher or lower than the under layer then draw in the wrinkles. You may need to use a fuzzy brush and/or some dodge/burn. Another rather quick but dirty method is to use the SL AV wrinkle file and use that as a bumpmap layer.

Thanks! I could never really see the wrinkles in the Appearance mode, but that was because I most likely turned off dynamic textures.

Oh, one more thing: I'm never sure which layer goes above the other. There's like a Background Layer, the layer that you're working on with the shirt, then say, another layer for the wrinkles or additions to the shirt. Where does the 3rd layer go? Above or below the shirt layer?

Layers always work the same way: each layer affects any layers under it. Think of the layers as if they were a stack of paper..... some sheets of regular paper, some tracing paper, some acetate film, some paper with holes cut in it, .....

If a layer is opaque and totallly filled with pixels, you can't see any layers below it. (Better put that one on the bottom -- it's the Background layer.)

If a layer has cutout holes in it, you can see the pixels in layers below. (This one's a mask, maybe, or it has applique pieces on it.)

If it is translucent, you see the layer below it like you're looking through colored glass. (This one's an adjustment layer, made so you can modify the color, contrast, or other properties of your design without permanently messing with what's on the opaque layers below.)

If a layer has a mode like "Multiply" or "Overlay," it multiplies or overlays all visible pixels in layers below it. (This one's where you use dodge and burn to create your shadows and folds, maybe, to give your design a 3-D look.)

Stack as many layers as you like, in any order that makes sense to you. All you have to remember is that your eye will see whatever is on top. If it can see through what's on top in any way, the stuff below will be modified by what's on top.

Does anyone know a trick that will allow to 'control' the direction / size of the creases in the gradient-blend process?

I think I may have confused things. The 'wrinkles' I refer to are found in your SL directory on your hard drive. They are a TGA file and are the wrinkles used when in appearance mode. You would use that as a bump-map layer to create wrinkles on the shirt. They are not the best 'wrinkles' around but they will do OK if you want something 'quick and dirty'. They can be made to look 'OK' but you'll need to play around with the bump-map height to get them looking somewhat realistic.
If, as I suggest you do, you make your own wrinkles then just create a new layer then on that layer use your skill and judgement to draw in the wrinkles and creases. I'd suggest that you keep that layer as a seperate file so as you can use it later on future clothes.

I have a tutorial on how to create "Wrinkles" but it isn't "Quick and Easy" but you will have more control over it.

here:
http://secondwavefashion.blogspot.com/2008/01/satin-tutorial-from-sioxie-legend.html

Even though I used Photoshop - I believe that Gimp should have the same capabilites. Hope this helps.