[Xperia Z Q&A, Help & Troubleshooting] [NUT]:
This i have written in a PM to someone needing help, thought it might be useful to more then just him, so here it is:

To download in preparation:

DooMKernel v18

The 2.67 FTF for either the C6603 or the C6602

The latest Flashtool

OPTIONAL: The files for my 'update to' package

The guide:

Put the DooMKernel zip on the external SDCard;

Install flashtool and put the FTF you downloaded in the X:/Flashtool/firmwares folder (where X is the drive you installed it on);

Create a folder called 'kernels' inside the Flashtool folder;

Inside X:/Flashtool/drivers you will find the Flashtool driver package, install the drivers for the Xperia Z;

Open the zip from doomkernel and extract the boot.img into the 'kernels' folder you just created;

Make sure you have a backup of everything you wish to keep on you phone,

this process will destroy everything on your phone

;

Connect your USB lead to your PC ONLY;

Turn off your phone, leave it disconnected for now;

Start flashtool, click on the lightning rod and choose 'fastboot mode';

Click on 'Select kernel to flash';

Browse to X:/Flashtool/kernels and change the file type selector to '.img'. Select the boot.img we put in there but WAIT with pressing OPEN;

Now connect your USB lead to your phone, while HOLDING the VOL-UP button;

Once you hear your PC register the phone (and maybe install a driver) press OPEN (This will flash DooMKernel to your phone);

Disconnect your phone from the PC when you see the Flashtool main screen say that flashing went OKAY;

Now turn on your phone, when you see the purple LED, press VOL-DOWN once, this will start TWRP;

Tap 'Wipe';

Tap 'Advanced Wipe';

Tap to Tic everything in the list except the external_sd, then 'Swipe to Wipe';

Once done, your phone will NOT boot, as it will be empty, go back to the main menu and through the menu option 'Reboot' choose 'Power Off';

In flashtool, close the fastboot toolbox and open the lightning rod again, now go to flashmode;

Select the FTF we put inside the 'firmware' folder;

Under 'Exclude' exclude the KERNEL, leave the rest as it is;

Press 'Flash' and wait for the screen where you get instructed to connect the USB lead;

Connect the USB lead while HOLDING the VOL-DOWN button;

You will notice flashtool will start flashing your device;

Once done disconnect the USB cable;

Power on the phone, at the purple LED press VOL-DOWN to enter TWRP;

Tap 'Install' and on your external_sd find DooMKernel, install it (This will install WIFI drivers and firmware);

Reboot your phone and let it boot;

Setup your phone walking through the basic settings and all (similar to when you first got it);

The following procedure is optional but will also root your ROM

OPTIONAL: Put the 'C660x.flashable.67.zip' you downloaded on the external_sd card;

OPTIONAL: Reboot your phone and enter TWRP again;

OPTIONAL: Tap Install, search for the flashable package and install it, do not reboot;

OPTIONAL: Install DooMKernel again through recovery, this will again install the WIFI driver and firmware;

OPTIONAL: Reboot your phone.

Be a happy camper, because you are DONE! :D

Long list of stuff to do, but it will work :fingers-crossed:

If you have any questions, do not hesitate to ask :good: