[Xperia Z5 Premium Q&A, Help & Troubleshooting] xirt0n:
What you want to do is follow sonys instructions on how to check if your phone is able to unlock bootloader here: http://developer.sonymobile.com/unlockbootloader/unlock-yourboot-loader/

If you can unlock your bootloader, which you will know after performing the simple steps provided by sony, then you need to install android studio along with required drivers etc, before you can actually unlock the bootloader.

Android studio can be found here: http://developer.android.com/sdk/installing/index.html?pkg=studio

Read everything throughly, it is important to get all the details regarding usage and installation.

After installing Android studio, you should download all the required SDK's found inside android studio->Configure->SDK Manager.

On the left side you will see several options in the navigation menu.

First go to Appearance & Behavior->System settings->Android SDK. Then in the "window" you have, make sure the first tab is selected (SDK Platforms).

Here you need to check all the android versions your phone(s) run.

Then go to the second tab (SDK Tools) and download:

Android SDK Build Tools

Android SDK Tools 24.4.1

Android SDK Platform-Tools 23.1

Google USB Driver

Android NDK

I am not sure if you really need the NDK one, but the rest I think is required. You can do as I did and choose all if you want.

On the last tab (SDK Update Sites) I have checked everything.

When you have done all of this, you should install Flashtool and go to the install directory and open the Driver folder, where you will find a .exe file. Run this and click next untill you can choose a few items from a list, in this list you should check Fastboot drivers, and USB drivers, and any other options that are relevant to your use, then finish the installation. (The entire installation of Flashtool should be done with 100% admin rights on the user).

After this is done, connect your phone to your pc and go to device management on your pc and you will likely find something similar to this:

If you have this yellow exclamation mark triangle thingy, then you will need to manually install your USB drivers.

Download your driver here: http://developer.sonymobile.com/downloads/drivers/

Then right click the yellow exclamation mark triangle thingy device in device management, and select update driver, then select "Browse my computer for driver software".

On the next window, select "Let me pick from a list of device drivers on my computer".

In the list that appears, select "Show All Devices" and click next.

Click the "Have Disk" button, then navigate to the driver you downloaded, and you will get 3 options in the window on you screen, select "Android ADB Interface" and click next.

Confirm by pressing "Yes" Then press "Install".

If completed without problems you should see "Windows has successfully updated your driver software".

*NOTE* On the above steps, you can also navigate to the folder where you installed android studio, which should be something similar to this: C:\Program Files(x86)\Android\android-sdk\extras\google\usb_driver OR it can be found your USER folder.

and here you can install the usb_driver aswell, which should make most phones support USB debugging and ADB on via your pc.*NOTE*

now reconnect your phone and the exclamation mark triangle thingy should be gone, good!

Next thing you can do is navigate to the installation folder of Android studio, and go to the Platform-Tools folder.

Inside the folder, hold down shift, while right clicking any white empty space inside the folder, and click open terminal here.

In the terminal, type: "adb devices" /Without the " marks. then it should say something about daemon not running etc, when its done, do the same command again, and you should se you device listed. Good, at this point you are ready to unlock bootloader!

If you have succeeded in every step so far, then go ahead and unlock bootloader following sonys instructions!

When you are done, you can simply follow the guides provided in the custom Rom thread itself most of the time.

If not, then a simple way to do it is:

Get all the files you need for the rom flash. That means Kernel, rom img, and a custom recovery like TWRP and SuperSU.

It is REALLY important that all files are for your device specifically!

Put the SuperSU.zip and the custom Rom.zip on your sd card and put it in the phone.

To avoid any trouble, do as I did and save all those files in the Platform-Tools folder, because you need to open the terminal as shown earlier in the post, by holding shift and right clicking in the same folder as the files, this is a must.

Open the terminal from the same folder as the rom, kernel and recovery.

type:

fastboot flash boot NAMEOFYOUR_KERNEL.img (This is usually boot.img)
fastboot flash recovery NAMEOFYOUR_RECOVERY.img (Usually contains TWRP in name)
fastboot reboot

You should now press volume up key when a led lights up while you see the sony logo, a orange light, not the red one, this will bring you into recovery, from here choose install, then navigate to your SuperSU.zip and install it.

Reboot your phone and you are now rooted, if you also have a custom rom, then place it on your sd card like the SuperSU.zip, but make sure the custom rom is a ZIP file!

Boot into recovery and install the custom rom, wipe cache and dalvick cache.

Congratulations on your custom rom! :)

Hope this helps a bit, sorry for any errors, but this is exactly how I did it, as far as I can remember!

Btw, this can seem like alot of work, especially when and if you encounter any problems while installing. I know I did encounter ALOT of them, but hopefully with my simple guide you can avoid them!

Luckily, this is just something you have to do once if you do it 100% right!