General :: Recompile System APK Using ONLY APKTool While Keeping Signature?

I don't want to use APK Multi-Tools or APK_Manager or whatever. I want to know exactly what those tools are doing in order to recompile the system apk while keeping the original signature.

What I initially did...

Code:
apktool d "%~dpnx1.apk" "%~dpnx1"

...modify files here...

apktool b "%~dpnx1" "%~dpnx1.apk"

But obviously that's unsigned. I need to sign it, but I want to use the original signature, which I'm guessing is the META-INF stuff, right? So I attempted this...

Code:
apktool d "%~dpnx1.apk" "%~dpnx1"
7za x -o"%~dpnx1-signature" "%~dpnx1.apk" META-INF -r

...modify files here...

apktool b "%~dpnx1" "%~dpnx1.apk"
7za a -tzip "%~dpnx1.apk" "%~dpnx1-signature/*" -mx9

You'll notice I now use 7-Zip to pull out META-INF from the original APK and then insert it back into the recompiled one afterwards.

The only other thing I can think of would be to go with my first method, but then use 7-zip to pull out the compiled files from the unsigned apk and insert them into the signed apk. I haven't tried that yet, but it will be the next one I go with. Every time this fails, though, I have to do a nandroid restore.

For those interested, after running "apktool b" (to build/compile), ignore the output zip. Instead go into the source directory and there is now a build directory. In that is an apk directory. Simply add your changed files to the original untouched APK (if you modified any XML files, then replace resources.arsc; if you added anything to smali, then replace classes.dex; if you added any XML files, then you need to include those as well; for me, i copied resources.arsc, classes.dex, and one new XML file). I used the following command:

Code:
zip -9 ..SystemUI.apk reslayoutstatus_bar.xml classes.dex resources.arsc

I use zip instead of 7za for cosmetic reasons. You can also use "7za a -tzip etc..." if you want.