This tutorial is a long winded way of doing pleats, I recommend doing it this way to start, but as your skills grow you will be able to refine this and make it quicker.
Bear in mind that ALL my skirts (except gowns) are based on the long skirt mesh.
1) Open your skirt template/mesh file
You'll notice on mine I've stretched out the leg mesh to show approx positioning of the final skirt.
2) Add a new layer and make a rectangular selection. This will be the base of your skirt, so choose the length you are after.
3) Select the bucket fill tool. Then below that, select pattern fill and select the pattern you want to use (mine defaults to pine), I chose the pink tarten pattern I made in the previous tutorial.
Go to your rectangular section and click inside it.
4) Mark out the centre and bottom edge of your skirt using the guides. To use the guides, click on the arrow in the rulers at top and left of the image and drag them into position. If you look at the bottom of the window you will see the position changing as you move it.
Only put guides in one 1/2 of the skirt.
5) Again choose the rectangle select tool and mark out a rectangle within one of your selected areas from above. Another benefit of using guides is selection tools will jump to the guide line when you get close.
Copy the selected area (ctrl+c) and create a new layer, then paste into the new layer.
Before anchoring the selection choose the perspective tool.
Click on the selection and drag one corner down a little. Once you are happy with the size and shape, click ok then anchor the selection.
Repeat this step for every area you have marked out.
6) Once you have completed step 5, duplicate each pleat layer you have created.
Then select the flip tool. Choose each of the duplicated layers and click on them. This will mirror your pleats.
7) Go to your left most pleat layer and right click on the layer (in the layer window) and select Alpha to selection
Then select Filters/Light and Shadow/Drop Shadow
The 1st thing to do with the drop shadow dialogue box (every time you use it), is uncheck the "Allow resizing" box, this can save you all sorts of cropping later on
Now adjust the offsets and blur radius (you can also adjust the colour and opacity, though default is fine for this tut).
Offset X - a positive number moves the shadow to the right
Offset Y - a positive number moves the shadow down
Blur radius - the larger the number the larger the shadow.
For this I had Offset Y at 2 and Blur radius 10 throughout and Offset X at +5 for the left pleats and -5 for the right pleats.
Once you have run the drop shadow on the left most layer, go to the next layer to the right and alpha to selection again. You can then re-run the drop shadow by pressing ctrl+f
8) Once all the pleats to the left of centre are shadowed, go to the right most pleat and alpha to selection as before. Go to Filters and you will see at the top "Repeat <last filter used, drop shadow in this case>" and "Re-show <last filter used, drop shadow in this case>". Select re-show drop shadow.
Then change your Offset X from + to - and click ok.
You can then use ctrl+f as explained in step 7.
9) Once you have finished all these pleats do a final save of your GIMP file (you have been saving as an .xcf file throughout, haven't you?)
Then hide all the layers you don't want to see, resize and save as a .png file (if this step confuses you, review my Basic Shirt Tutorial for GIMP).
Then you should be ready to test and publish your skirt, baring any additions such as buckles etc