[Xperia Z3 Q&A, Help & Troubleshooting] sapphous:
Hey Jstar22! The fact that someone might reply to your post was one of my beacons of hope as I searched the internet for this same information, a process which took several hours. So let me summarize it for you here. To be clear for everyone, this will work just fine on a locked bootloader; there is no need to use flashtool to unlock your bootloader in order to downgrade--if you get errors, that isn't the cause.

Edit: The XperiFirm part of this should be unnecessary if you get an ftf file of the version you want from elsewhere (say from the thread in the General section here). In this case you can skip steps 1-3 and 6-7 and modify the other steps so that you look up the ftf file.

1. Download&Install XperiFirm. A google search should bring it up easily; it's the same for all Xperia devices, so no fear of getting the wrong one.

2. Run XperiFirm. Navigate to your device model on the left. If you're on a DD6603 you probably want to sort my Market and select the only one labelled GLOBAL. When you click it the version should display on the right (should be 23.0.A.2.93; that's the one you need). As I understand it, the root exploit should work with options other the GLOBAL one too if you have a need for one, but I can't personally guarantee it. Click the version number and a dialogue will come up; uncheck "Unpack automatically" and download. If it bothers you about whether you really don't want to unpack, tell yes, you really don't want to unpack.

3. When the download is complete, you will have a folder with 4 files. Hooray!

4. Now, download&install flashtool. The internet seems to love saying this but doesn't bother to specify that it's not Sony's flashtool you want; it's the flashtool from flashtool.net.

5. Once flashtool is installed, you will have an icon for it in your start menu. By default it installs to C:\Flashtool on Windows; on Linux it will be wherever you saved it when you downloaded it. Run the program and wait for it to finish loading.

6. Go to Tools->Bundles->Fileset Decrypt. Click "..." and navigate to the folder from XperiFirm, containing the 4 files. They will appear in a menu; select them all and click the right arrow; they'll all be on the right side now. Hit "convert", and wait for it to finish.

7. A window will pop up. Double click the grey rectangle by "Device" and select your device. Put "23.0.A.2.93" for the version and "Generic GLOBAL" (or whatever you downloaded, if it wasn't the GLOBAL version) for the branding. Select all the files in the box except for the .ta files (there are only two of those), and click the right arrow. Then click "Create".

8. Make sure you have Unknown Sources (in Security) and USB Debugging (in Developer Options--if you don't have Developer Options go to About Phone in Settings, and click Build Number 7 times, and you will then have a Developer Options area in Settings) enabled. Turn your phone off. While holding the down-volume button, plug it in via USB to your computer, while flashtool is still running; flashtool should say that a device connected in flash mode. Do not let go of the down-volume button. Click the lightning bolt and select "flashmode". Select the firmware version we set up before ("Generic GLOBAL" or whatever"). Click "Flash". Don't let go of the volume down key until it's begun flashing (otherwise you may get an annoying issue where your phone "disconnects" and turns itself on). If it fails with an error about not being able to delete a file in C:\Flashtool\firmwares\prepared (or wherever you installed flashtool), delete the "prepared" folder there and try again. That was necessary for me.

9. You win. A quick Google search will bring you to the rooting thread, where bliss and long-awaited joys will overtake you.