[AT&T Samsung Galaxy S 4 General] StoneyJSG:
FAQ section in the 2nd post.

BEWARE OF EVERYTHING! I AM NOT RESPONSIBLE if your phone implodes, aliens take over Mars, don trump starts WW3 and WW4, or your dish washer starts working again from using this guide. Remember: just because it worked for me, does not mean it will work for you, but it might! I recommend having an ODIN copy (.tar file) of the MDL bootloader so in case anything goes south, you can get back to stock and start over from scratch.

IMPORTANT! This guide will show you how to (easily) maneuver around the At&t sgh-i337 on the MDL bootloader as far as root and other things are concerned. This guide is NOT for any other bootloader! Once you have updated past jelly bean 4.2.2 then you will not be able to use this guide!

Now for the good stuff...

So you've just taken your new At&t GS4 out of the box and discovered that you're on the MDL bootloader (like I did)! First you're going to want to root it. You can do this by grabbing a good 'ol copy of towel root (just search the forums here or Google for it). Next, you're going to want to freeze the OTA update so the phone won't update off the MDL bootloader (after installing SuperSU of course). You can do this via the Titanium Backup app or the MyBackup Root app (now called just MyBackup I think).

Let's recap...you will need a copy of the towel root app, then SuperSU, then TB or MyBackup Root app. SuperSU will kill the evil KNOX for you.

Next you may want to install a custom recovery. To do this, you will need to use an exploit file called Loki Doki. Follow this video to take the needed steps to get loki'ed and a recovery installed: https://www.youtube.com/watch?v=VmkL9atpVHw

Yes, I know it installs CWM which is way out of date, if you want to use TWRP go ahead. However, this guide deals with CWM so if you use TWRP you're using it at your own risk and going against the guide! GO WITH THE GUIDE!

However, if you want to update to TWRP recovery then this video by the same guy as in the last video should do it: https://galaxys4root.com/galaxy-s4-...or-verizon-galaxy-s4-with-twrp-v-2-8-4-0/amp/

Once you've got loki lurking around your phone's memory, you're good to install a custom ROM! TA-DA! Awesome how all that worked out isn't it?

How to install a custom (TW) ROM

If and only if you followed the ZedoMax video, you should now have OUDhs CWM Touch Req0very v1.0.3.4 installed. Now would be a great time to boot into it so power off your phone then hold power, home, and volume up at the same time. Hold it! Hold it! Hold it! Release when you see the CWM menu. Now let's Nandroid this hush puppy!

Goto backup and restore and hit backup. It will most likely save it to your internal memory, but you can copy it to SD card later, or just reboot now and move it. We now have a working backup in case your dog eats your homework. After that's done, make sure the ROM .zip file you are going to boot is on your SD card. Now go to wipe cache partition and select yes. Go to Advanced and wipe dalvik cache. Go to wipe data/factory reset and select yes. Goto mounts and storage and select format cache, then select format /system.

NOTE: If you want to keep all your personal data (pics, videos, etc.) then stop here! If you want to completely nuke your phone and start from scratch then select format /data from the mounts and storage menu.

Once you've formatted /system (or did a complete wipe by formatting /data too so now you have nothing), you're ready to install a ROM. Select install zip from SD card from the main menu. Find your ROM .zip file and select it. I used the latest GoldenEye ROM because it has an Aroma installer built in so it pretty much does everything for you once you tell it what you want. Easy!

ROM should now be installed and everything should work ok once your phone reboots out of CWM recovery.

How to boot a custom (AOSP/CM) ROM

WARNING! THIS IS EXPERIMENTAL! Following this may get your ROM booted or not. Do so at your phone's own risk! You could erase stuff on accident!

Place the ROM file on your SD card. Boot into recovery and select wipe cache partition. Then select wipe dalvik cache. Now select wipe data/factory reset. Once all that's done, you will need to format /system and /data. Now goto install .zip from SD card. Select your ROM file and pray to whatever higher power yourbelieve in that it worked. Next you're going to (maybe) need a kernel.

Kernels and Modems

Download a compatible kernel for the ROM you want to use. Boot into recovery and flash the .zip file for the kernel from your internal memory or external SD card. Be sure if you are booting a TW ROM that you use a TW kernel. Same goes for CM and AOSP.

Modems are pretty much universal on the MDL firmware and possibly on others as well. To flash a modem just select the .zip file and flash in recovery from your SD card or internal memory. You can use any baseband modem...MDB, MK3, NB1, OC3, etc.