GIMP has most of the functionality that photoshop has. My PS is an older version (5.1) so GIMP actually can do some things my PS cannot, such as word art and defomations, etc.
However, the same general functionality is there.
You have a polygon select tool. You have a paint bucket tool. You have a brush tool.
You can add multiple layers. You can change the color more.
The difference is only where the menu or the toolbar is.
The steps are still the same: Set up the directory, get the default files, open the default files, add layers on top, in those layers create your own skin piece by piece, and then flatten the end product and save it out as 256-color (8bit) BMP.
I didn't want to create a guide to the software, as much as a guide to the overall process. There's so much subjectivity and creativity when using a graphics program. You have 10 people do a task, they could do it 10 different ways (and every one of them be right in doing so) and get the same end result. I didn't want to get into how to use the software. I leave that up to the person creating the skin.
I would say play around, get familiar with the software. Then as you go through the tutorial I typed up, just think "Okay, he says this, how do I do that in GIMP? Where is that tool?"