Turn on the phone in Bootloader Mode. Hold down the Up arrow and Power button at the same time until it gets to the Bootloader screen. Then connect the phone via USB to your PC
Open RSD Lite
Browse to the SBF File by clicking the ... button
Locate the file then select Uncompress and start Flashing. Or you can select Uncompress only. Then when it is done, click on Start
The phone will start flashing. This could take awhile. After it is done, Reboot.
If you want to completely erase the phone:
Go to Settings->Privacy->Factory Data Reset
Click Erase everything
After the phone reboots, you will get the Activate screen