Installing CyanogenMod 12S Official build:
Copy the ROM zip file to your phone(By using a USB cable or via adb push command).
Enter into recovery mode by pressing your device specific key combinations.
Or else if you are on a pc, then type the following command:
$ adb reboot recovery
Wipe system partition, cache, data (Factory Reset)
Select the Install Zip from SDcard option.
Select the CyanogenMod 12 ROM file, let the installation complete and then reboot.
That’s all guys. Cheer up and have fun.
OnePlus One forum link