[T-Mobile, Samsung Galaxy SIII] Kilokk:
Well I can help you with that... It is a bit of work, but I have been successful on the process.

Prerequisites:

Install whichever TWRP (TWRP 2.7 a.k.a. d2tmo-twrp2.7-mb-v2.11.zip) is the one I prefer.

Place ROM zips and GApp zips on the root of your external sd card.

> Flash your new recovery from your current recovery, then reboot into recovery.

Once you have your new multiboot recovery installed and the ROMS you want to have installed on your SD Card (Note: each ROM creation will use about 6.5 - 7 GB of your SD Card) you're ready to create a ROM.

1. Select the Advanced Tab from the main menu

2. Select multiboot

3. Select Create ROM

4. Name your ROM and set the size you want for each partition, if you are unsure of how much space the ROM will consume with already installed apps, data and cache, you can leave the preloaded values as they are. The amount of apps you install, the more system, data and cache space you will need, but the predetermined sizes will work as if you had a new 8GB phone without an SD Card slot...

5. Swipe the bar to begin the process... it will take quite awhile for it to configure the "Virtual Machine" so be patient.

6. Once the process has finshed, select BACK to get back to the multiboot menu

7. Next select "Set ROM", and select the ROM you just created, then swipe to confirm, you should now see at the top of your screen in (RED) ROM: Your ROMs Name

8. Next go to the TWRP home screen, (Bottom left icon) to get back to the TWRP main menu. Now you are ready to install your ROM.

7. Select the directory where your ROMs are ready to be installed and GApps if needed are.

9. Choose your ROM and swipe to install, this will also take a while, about the same time as normal, maybe a little longer.

10. Once the ROM is installed do the following: Go to TWRP main menu, select "Advanced", then "Multiboot", "Set ROM" (the button now says "Reset to Internal" select and swipe. When it's done hit back (takes you back to Multiboot main menu) now select "Boot ROM", select the ROM you created swipe, then you will see "Reboot New ROM" select you you will reboot into your Multiboot ROM.

Your original ROM you had working before you installed the Multiboot TWRP will still be intact and is referred to as your Internal ROM. When you boot into your multiboot ROM changes you make do not affect your Internal ROM. So if you just do a normal reboot, you will reboot into your internal ROM. Also, if you install a ROM that requires you to flash GApps, you can either flash them while you are in recovery flashing the ROM and your recovery is still "Set" to that ROM, or if like in the video you boot without installing GApps, you will need to reboot into recovery and follow the steps to "Set" the ROM you are working with, then install GApps, flash a new kernal, or mods whatever you want, the reset to inernal and select what ROM to boot and you are good to go. I have 4 ROMs currently I am working with. SOKP for my internal ROM, PAC, IACP, and Resurrection ROM for my multiboot ROMs. They all work fine including the ones I flashed kernels and other mods as well. PM me if you need further help or hit me up on Hangouts if you or anyone else that needs help as well.

michaeldetaranto@gmail.com

If I made a mistake in the steps let me know and I will update them... at work on a break so sorry if anything didn't make sense.