[Sony Smartwatch 3] KittenSauce:
Step 1. Go to Settings on the watch, scroll down to "About Watch" (or was it just "About", whatever), tap on it, tap on "Versions", and tap on "Build Number" about seven times to enable developer settings.

Step 2. Back out of "About Watch" and tap on "Developer Settings". Enable ADB.

Step 3. Install the Android SDK on your computer (it's on the same site as Android Studio, just scroll down to "Command Tools Only", or whatever it's called), along with platform-tools and build-tools (both can be downloaded from the Android SDK GUI, once it is installed).

Step 4. Connect the watch to the computer.

Step 5. Open CMD or Terminal, depending on your OS.

Step 6. CD to wherever you install the Android SDK, CD to "platform-tools", and type "adb devices" and accept the request on your watch. Hopefully your drivers are working. If your drivers aren't working, check here.

Step 7. If your watch shows up, type "adb reboot bootloader". You will reboot to fastboot.

Step 8. If, by some miraculous chance, your fastboot drivers are working, your device will show up when typing "fastboot devices". If your drivers don't show up, follow the link previously mentioned in Step 6.

Step 9. WARNING: THIS WILL DELETE EVERYTHING ON YOUR WATCH. Type "fastboot oem unlock" twice. Your bootloader is now unlocked. You will never have to unlock it again. But wait! There's more!

Step 10. Type "fastboot format cache", followed by "fastboot format userdata", and finally "fastboot reboot". This is to actually make an unlocked bootloader function properly.

Step 11. Set up your watch as normal and enable Developer Settings and ADB again.

Step 12. Download the ROM and TWRP from this thread to your computer.

Step 13. Connect your watch to your computer, enter Terminal/CMD, and type "adb devices". Accept the request once again (or maybe it doesn't even come up? I don't remember).

Step 14. Type "adb reboot bootloader". Good luck with those drivers.

Step 15. Once in fastboot, type "fastboot devices" once again to ensure your device is properly connected. If not... you're in some hot water.

Step 16. Type "fastboot flash recovery", then a space. Drag in the TWRP .IMG to CMD/Terminal you downloaded from this thread, and press enter if the command doesn't automatically start.

Step 17. Type "fastboot boot", then a space, and drag in the TWRP .IMG again. This is to stop stock recovery from automatically overwriting it.

Step 18. From TWRP, reboot to System.

Step 19. Connect to your watch via ADB again, this time typing "adb reboot recovery". You should reboot to TWRP.

Step 20. In TWRP, ensure your ADB drivers are working once again by typing "adb devices". Your watch should show up as your serial number, then "recovery".

Step 21. Type "adb push", then a space. This time drag in the ROM you downloaded. Then enter another space if one is not already inserted, and type "/sdcard/". Finally, press enter.

Step 22. Once transfered, go to "Install" in TWRP. Go to the "sdcard" directory if necessary (I don't remember if TWRP automatically opens to that directory) and select the ROM.

Step 23. Flash the ROM! Afterwards, wipe Cache/Dalvik (I think it's an option once the ROM is done flashing).

Step 24. Reboot to System, and you're done! Have fun!