[Galaxy S 5 Q&A, Help & Troubleshooting] ag4751:
Extract the framework-res and twframework-res apks and put them in the folder with the SystemUI.apk

Open a command prompt in the folder and type apktool if framework-res.apk and hit return, then do the same for the twframework-res.apk file, this will generate the resource files needed.

To unpack an apk type apktool d SystemUI..apk for example hit the return key, to repack type apktool b SystemUI (usually the decompiled apk folder doesn't have the .apk after it), this will generate a new apk

Copy the Meta.inf, Android Manifest.xml and resources.arsc files from the original apk to the new apk , make sure the apk is titled exactly the same as the original apk so SystemUI.apk not systemui.apk and the new apk should work.

One thing you can do if you are just changing the png files is extract the png files from the folders direct from the apk to a working folder, create your new png files so they are the same size and exactly the same name as the originals, drag and drop them from the folder directly back into the folder location in the apk then reinstall the apk on the phone, this just replaces the png's, you only really need to decompile the apk if you are making changes to the xml files.

Use 7zip its better for these than winrar (personal opinion), if you want to male changes to the xml files then use Notepad++

Always keep a copy of your original apk's as a backup unless you want to install the firmware again should something go wrong.

EDIT:-

Thought you should have a bit more info about apktool, the red text is the apktool commands you would type in the CMD window from your working folder

Decompiling should look similar to this

C:\Samsung S7\APKs>apktool d SystemUI.apk

I: Using Apktool 2.1.1 on SystemUI.apk

I: Loading resource table...

I: Decoding AndroidManifest.xml with resources...

I: Loading resource table from file: C:\Users\apktool\framework\1.apk

I: Regular manifest package...

I: Decoding file-resources...

I: Decoding values */* XMLs...

I: Copying assets and libs...

I: Copying unknown files...

I: Copying original files...

C:\Samsung S7\APKs>

When you recompile an APK it will be located in apk folder your compiling from so you decompile the SystemUI.apk which will become a folder called SystemUI when you re-compile the SystemUI folder to make a new apk then the new apk will be located in your SystemUi folder in a folder called dist.

When you recompile an apk it should look similar to this you can see here also a warning message you can ignore these they wont affect how the apk works provided you copy across the original files from the old to the new apk as mentioned below

C:\Samsung S7\APKs>apktool b SystemUI

I: Using Apktool 2.1.1

W: Could not find sources

I: Checking whether resources has changed...

I: Building resources...

W: warning: string 'tw_screen_pinning_negative' has no default translation.

I: Building apk file...

I: Copying unknown files/dir...

C:\Samsung S7\APKs>

Using 7zip open this apk and open the original apk and simply drag the META.INF, respources.arsc and Android Manifest.xml from the original apk to your new apk close both apk's, put the new apk on your phone.

If you ever make any changes in the decompiled values folder then when you recompile your apk and copy across the above files from the old apk don't replace the resources.arsc file as thats where the values folders are just replace the Android Manifest and META.INF files.