This tool has ADB and FASTBOOT all ready to go so you do not have to download android sdk and learn a bunch of new steps just to flash fastboot images.
Download the Boot Image Flasher
extract to the desktop
place the boot.img from the rom you are installing into the folder of the extracted files
plug in the phone and run the RUN ME file, There are 2 files, One is if you are currently in android and one is if you are already in fastboot.
Hit enter after you run the file and let it flash the boot image, after that you can flash the custom rom