If you read this FAQ, it advises how to set up your PC to use fastboot and ADB.
Once you have done that, the fastboot commands are input in cmd (command prompt).
The command you will want is:
fastboot flash hboot c:\bravo_alphaspl-oxygen.img
for example, providing the file is called that and is on the C:\ drive and not in any folders.
Your phone needs to be in fastboot mode too. This is all explained in the FAQ, except your specific command of course