Glad to be of help
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The installation of the plug-in is very simple. If you are using Windows you should have downloaded the according version that only includes one executable file. And this one goes to:
[installation directory of gimp]\lib\gimp\2.0\plug-ins\
After just copying it there and (re)starting gimp, you can just load those dds files like any standard image file format. On loading a dds file the plugin will ask you whether or not to load mipmaps. I didn't load them, on saving you can just let it create new mipmaps.
If you loaded a texture and now want to edit an emblem onto it, you would do the following steps.
The top menu bar (of the image window) shows an option 'dialogs'. There you find the layer dialog - open it. You will notice that there's currently only one layer there - the background layer. In case of our sh4 texture it will be called 'main surface'. By right clicking onto a layer in this small window you get a whole lot of options. One important would be 'create new layer'.
Choose it and create a transparent layer. It will automatically be the same size as the image, but can also be resized if needed.
The very convenient thing about layers now is, all the drawing, adding or whatever goes on a newly created layer (or even more than one later on). By clicking on the eye symbol besides it, you can switch a layer off and on.
How it might look this far:
There are actually 2 'main surface' layers here, but that's just something of my previous work.
If you now save this state of the file in the gimp file format (.xcf) every layer with all it's set options will be stored as is. This allows you to simply continue your editing later on or, even more importantly, lets you use previous work easily for newer projects.
If you have multiple images already opened with gimp, you can use select tool and just copy&paste stuff between those images. Also very useful yet nothing very special
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When you want to save your work as an dds file again though, you have to do some stuff beforehand. First, you can only save an image as an dds file with this plugin if the image has only one single layer. So you should save your work as an .xcf file (just for having it saved) and then do the following.
Activate all layers that contain stuff that is supposed to be on your final texture. Then you right-click any one of those layers in the layer window and select the option 'merge visible layers'. Then all those currently visible layers become a single one that contains everything that was visible when you did that. All layers that weren't visible still remain. (I often have a lot of layers with temporary work around). Before you can now save as an dds file you gotta delete all these unused layers first. Then you have only one layer left that contains everything the texture should contain.
One thing that still might happen if the layers got moved up/down, left or right during editing, or if they weren't the same size before is that you see a thin, yellow dotted line around your image that contains an area larger or smaller than your texture image. This only shows the layer boundaries, but which have to be exactly the same size as the overall image for the dds saving to work. (actually not the saving itself but for the usage of a compression algorithm).
You can establish the right layer size by right clicking on your single remaining layer and choosing 'layer to image size'.
Save it as an dds file and the plugin will present you a dialog where you can have it generate the mipmaps again and choose a compression algorithm - sh4 uses the dx1 type.
That should be the meanest pitfalls you shouldn't fall into anymore now.
I hope you have fun editing and we see your work here soon
