Alex, link this in your OP
knuxrouge's comprehensive tutorial on character textures for gimp users.
Try saying that 5 times real fast.
Prologue, your vision
It's important to know in advance what you're going to make, but if you're confident and feeling lucky, you can freestyle if you please, but consider the consumer value of it, assuming you plan on sharing it (lol naked zss), you have to make sure that it appeals to others, in doing so, you may have different versions of the same texture, as you plan to improve it later on. Bear in mind what others think of your work.
Step 1, your materials.
First and foremost, you'll need the proper items to get started, everything you need should be in the OP, save for two other things that cannot be discussed on acquiring, sorry. Now, you know to have ash v23 set, now you just need a character to fondle with. Pick your character, with that character is a zip file containing all their alternate costumes and their Entry and Final Smash textures, since I never tested Entries and Final Smashes, lets skip those.
Extract the zip file and select the pac file that you think would suit your editing ease better, for example if FitGuy03.pac is a closer match for your idea of a tuxedo wearing Guy, then so be it. Once you have your pac file, it's time to set up your workstation.
Step 2, your workstation
Being a person who not only does textures, but composes music and makes flash movies, I can personally vouch that having an organized workstation is ideal for any kind of project, things will flow better as you'll know where all your assets are, where and when are they necessary. But the ultimate question is, what does a coherent workstation comprise of?
Well, there is no set way to organize your items, but there are some key elements that helps run things along smoothly.
1. Reference.
Chances are, you'll be modeling your texture after a pre-existing person, whether fictional (Robin) or real (Obama), in that case, you'll need a reference picture to make sure your details are as accurate as you can make them, what goes where, what is colored how, that whole deal. You won't need a full 3D model, just something to give you a general idea of what to do, it'd suck if you intended to make a Johnny 5 texture for ROB and instead made him look like Wall-e's defective prototype. Make sure you have a reference to base your work off of.
Another reference you'll need (if you're inexperienced) is a reference for the original character model, this will help you know what part of their jumbled texture is what on their body. That helps you not misplace an edit.
2. Side pictures.
You may want to add logos and/or pictures in your texture, it's wise to have them prepared before hand, perhaps in a new gimp window, note that most character designs are mirrored, meaning that their texture construction is just one half of a body part that is mirrored in real gameplay, the perfect example would be most character's facial construction. This is relevant, because sometimes, to place an image correctly on a character, you'll need to cut it in half and place it correctly in the right spot, make sure it doesn't interfere with other parts of the body. This also limits what kind of pictures you can properly put on a character, especially if you want it to be centered on their body somewhere.
3. Back up
Lets say you screwed up a texture, but you already saved the document, touchy predicament, right? Not if you back up all of your completed, untouched or undone textures in a separate folder, this way you'll be able to go back and try again from the start, be sure to replace the f'ed up texture folder with a copy (not cut) of your back up, you got yourself a new second chance, and infinite amounts of them that does not require having to extract the texture again.
With these things, you already have a healthy and organized workstation!
Step 3, the newbie step
You can skip this step if you already know how to get started with everything.
Basically, if you're new to this, you may want to follow this step closely, also beforehand, make sure you have your workstation set up.
With your pac file, just put it in your ash folder, now run "step1_extract", you'll be prompted to type in the pac file you want extracted, be sure to type it in exactly as the pac file is named (extension included), if you fail, the program will just close itself.
Upon success, you just need to play the waiting game, depending on your chosen character, the extraction will take some time, as some characters have alot of texture sheets (Peach, Marth), and some don't (Kirby, ZSS). Afterwards, once it's done, which you'll know once the program closes after being open for so long, open up GIMP, make sure you have the updated version. I use version 2.4.2, just make sure you have at least this version.
Step 4, your modifications
Once Gimp is running, open your preferred tga. image, for editing.
If this is your first time, you will be absolutely confused, but don't fret.
You may want to recolor or redesign a part of their body, the actual modifications are entirely up to you, but the select tool can be a great help, especially when you want to edit a specific part of the sheet. Selecting by color, shape or whatever, be sure to adjust the threshold (tolerance) to your liking, the lower the threshold, the more strict your selection process will be. This is a good thing if you're looking to be right on the mark for a texture. Before you save, if the texture name does not end with a +1 or a +2, you may want to add in an alpha layer, so go to Layer > Transparency > Add Alpha Channel, and you're all set, you'll always have to do this for new textures, these are called "normal textures", that terminology will come in handy later. Once your done, File > Save (not Save as) and move on to the next texture.
Step 5, +1 and +2
If the name of your current texture sheet ends with a +1 or a +2, then you may want to pay attention here. These are known as "indexed textures" and are a ***** to deal with if you don't know your stuff. For one, indexed textures need no alpha channel, so don't add one. Second, they are called "indexed" for a reason, instead of adding an alpha channel, you have to change the color mode of it from default "RBG" to "Indexed". Some textures have a pretty low index, but most with clothes are a standard 256, when running "step2_create", you'll be notified of the maximum number of colors in the palette, make the proper adjustments, to index a tga. go to Image > Mode > Indexed, and adjust the palette as such.
"But what about Transparency?"
Glad you asked, unfortunately, I messed around with that, but came up empty, transparency is a luxury us Gimp users cannot afford. Normally it simply requires pure Magenta (Red 255, Blue 255, Green 0) rubbed across whatever you want to render invisible, but because of Gimp's limited and makeshift abilities compared to photoshop, this just messes things up, for now, until a solution is discovered, leave that alone. Once you're done editing and indexing, save the file.
Step 6, wrapping things up
Now that you successfully done all your edits, it's time to make the new pac file, just simply run "Step2_create", it will first add your indexed textures, it will prompt you for each one, showing the maximum number of colors allowed in the palette and asking you if you want to include this texture, press "y" and proceed, after all the indexed textures are in, it will continue with the normal textures, you are not prompted on this, so just sit back, or grab a bite to eat, or go to the bathroom or something, once it's done, the program will close and you'll have two new additions to your ash folder, a "_new.pcs" and a "_new.pac" Rename them to the proper character and costume you wish to replace and put them in the respective folders of your SD card. Load up Brawl and take a look at your creation, from there is all you, you can choose to go back and re-edit some parts, or keep it as is and prepare for distribution.
Step 7, teh intrewebz
If you want to put your textures up for others to use, you'll need a hosting site to do it on, most of the time you can just hit up mega upload or media fire and upload your texture there, after that there isn't much left to do, your uploading storage time may expire and you'll need to re-upload it, be aware of that.
Issues? Please! (Troubleshooting)
Q: Halp, errournous colours!
A: This more than likely happens with indexed textures, this is a problem that lies in Gimp, just press "n" when prompted to add the texture, I'm afraid that's all you can do.
Q: WHERE DID MAH FOLDERS GO, HAXX!
A: Bear in mind when you run "Step1_extact" all folders will be deleted as to not mix any of them up, this is another reason to keep important folders out of your ash and have back ups of your notable items.
Q: This Troubleshoot sucks, there aren't enough answers to my questions!
A: Feel free to suggest questions for me to answer, but know that I'm no genius and I may not be able to answer your question or solve your problem the way you would want me to. Also note that I may not know how to solve your problem.