Let's start small and simple. Backgrounds are the most common thing to change, so let's begin with that. This tutorial assumes you are somewhat familiar with making mods. So I won't be over explaining the easy to understand stuff.
Step 1: Create a mod project using VisualMunge(found in BF2_Modtools\data\Build), or if you want to use an existing one, just ignore this step.
Step 2: Create a folder called shell in Data_ABC. ABC will be the Mod ID used in this tutorial. Replace all references of ABC to your Mod ID.
Step 3: Create a folder called textures inside of your newly created shell folder.
Step 4: Create (or copy) a req file inside the shell folder. Name (or rename) it to shell.
Step 5: Now let's choose the image we want to replace. Go to BF2_ModTools\assets\Shell\textures\PC and copy the profile_manager file into your Data_ABC\shell\texture folder. Make sure to copy the .option file for it as well.
Step 6: Now open your shell.req up. Change it so that it looks like this.
Step 7: Now the fun part! Let's change the texture now. Open up the profile_manager texture in GIMP, Photoshop, or PAINT.net depending on what you use. It will look like this.
Step 8: Now we're ready to munge. There's a couple ways to do it, but I personally find it easiest to just manually munge it, rather than use VisualMunge. The munge.bat you need to use is located in _BUILD\Shell.
Step 9: Now let's edit the addme.lua found in Data_ABC\addme. Add this line
Save the addme.lua. While you're there, also munge it.
Step 10: Now go to your addon folder, if you've ever installed a mod, you know where it is. Make a new folder called ABC (remember to change it to reflect your Mod ID).
Inside your newly created ABC folder, copy your addme.script file (found in Data_ABC\addme\munged) here. In the same ABC folder, create a new folder called data. Now inside of this data folder, create yet another folder called _LVL_PC.
Step 11: Copy your munged shell.lvl found in Data_ABC\_LVL_PC to your addon\ABC\data\_LVL_PC folder. All in all, the directory should look something like this. C:\Program Files (x86)\LucasArts\Star Wars Battlefront II\GameData\addon\ABC\data\_LVL_PC\shell.lvl
Step 12: Test it out! If you followed all the instructions, your new background should appear like this.
If you see any spelling/grammar issues, send me a PM and I shall fix it.
I'll also make a separate tutorial, or rather an extension on this one, on how to change all the buttons textures and stuff. That requires a common.lvl, but the process is pretty much the same.