How to "internalize" iPhone 6 running iOS 12

Requirements:

• Mac or MacBook
• Chimera 1.6.5
• Legacy iOS Kit
• InternalUI build (16C49)
• FTP client like FileZilla

Important:

• This guide, will work only on iPhone 5s and 6.
• There is a high chance of getting errors while transferring files via FTP from dump to iPhone (eg. WebCore.framework), don't mind them.
• While transferring internal files skip files that already exists, DO NOT OVERWRITE ANYTHING.
• Before starting, install any tweak that requires PreferenceLoader.

Steps:

1. SSH Ramdisk with Legacy iOS Kit and mount filesystem by executing "/usr/bin/mount_filesystem".
2. Transfer "/AppleInternal", "/System/Library/PrivateFrameworks", "/Pearl" and "/Perl" folder from dump to "/mnt1".
3. Transfer "/System/Library/CoreServices/SpringBoard.app" files except SpringBoard executable from dump to "/mnt1/System/Library/CoreServices/SpringBoard.app".
4. Add keys to "/System/Library/CoreServices/SystemVersion.plist":
   <key>ProductType</key>
   <string>Internal</string>
   <key>ReleaseType</key>
   <string>Internal</string>
5. Write file ownership and permissions by executing:
   chown -R root:wheel /mnt1/AppleInternal/
   chown -R root:wheel /mnt1/System/Library/Perl/
   chown -R root:wheel /mnt1/System/Library/Pearl/
   chown -R root:wheel /mnt1/System/Library/PrivateFrameworks/
   find /mnt1/AppleInternal/Applications -type f -exec chmod 0775 '{}' \;
6. Reboot
7. Re-Jailbreak and execute "uicache -a -f -r".

Thanks to Zielakless

SSH Ramdisk and InternalUI Dump Setup

1. SSH ramdisk with Legacy iOS Kit and copy "/AppleInternal" to "/mnt1".
2. Copy "/System/Library/PrivateFrameworks" from the dump and paste it to "/mnt1/System/Library/PrivateFrameworks" (skip things that already exist, we only need internal frameworks).
3. Copy Perl and Pearl (if they exist) from "/System/Library" into "/mnt1/System/Library".
4. Edit "SystemVersion.plist" to add the following:
   <key>ReleaseType</key>
   <string>Internal</string>
5. Save the plist and place it into the "CoreServices" folder.
6. Show the contents of "SpringBoard.app" in your dump and copy everything EXCEPT the SpringBoard (Unix executable file). Paste it into "SpringBoard.app" in "/mnt1", skipping files that already exist.
7. Run the following commands in SSH Ramdisk:
   chown -R root:wheel /mnt1/AppleInternal/
   chown -R root:wheel /mnt1/System/Library/Perl/
   chown -R root:wheel /mnt1/System/Library/Pearl/
   chown -R root:wheel /mnt1/System/Library/PrivateFrameworks/
8. Reboot and enjoy the internalUI dump!

NOTES: To internalize your device, it must be jailbroken semi-untethered (e.g., Taurine or Chimera). Semi-tethered jailbreaks (like checkra1n) will not work with this method. Be sure to install tweaks at least once.