(I'll make the guide look better when I get home)
Well lets get started.
First. An introduction to what an apk is. An apk is (believe it or not) a zip of files.
So how do you go about editing a zip? I suggest using 7z on your computer or root explorer on your phone.
Second: there are a lot of apks you can edit but for this guide I'll be using the systemui.apk that you find in any rom in the system/app folder.
Ok. So assuming you start with the system ui apk your going to need to extract the apk with 7z on your computer. (or root explorer on your phone. Es file should work also)
Now you should have a new folder where you chose to extract your apk. Open that folder. Your going to see a bunch of files. What your probably going to be interested in is the res folder. Go into that folder and find the drawable hdpi folder. Open it.
Your going to see a lot of icons. Here is where you can change things like wifi icons. You can add 4g icons and so much more. Even edit what your contacts look like.
This is an important step. Make sure your using the system ui apk from the rom your currently on or make sure its compatible with yoir rom.
Ok so now we get to the things like icon changes. Download a theme you've been dying for but couldn't get because it was either for another rom or phone. And the only thing you wanted was a few things from the theme.
Now extract that zip. Then extract the systemui.apk again from the zip you want your icons to look like. MAKE SURE YOU RENAME THE THE SYSTEM UI TO SOMETHING DIFFERENT SO YOU DON'T MIX THEM UP.
So now that you got the system ui apk with whatever you want to use all you need to do is go into the drawable hdpi of the one with the icons you want in it. Copy them and bring them to your roms stock system ui and replace them.
Thats about it. Now zip your roms stock system ui back up and change the ending to .apk. Then put it in system/apps and replace it. Reboot. And boom. Your modded apk is ready.
Hit thanks if I helped.