First off you need 3 things to sbf.
1- the latests Motorola drivers, downloadable from their website. Just do a Google search for Droid x drivers in Google to get to the page.
2- rsdlite (at least 4.8 or higher)
3- the full 2.3.340 sbf file
Both #'s 2&3 are available at mydroidworld.com under the Droid x discussion section, they are in a sticky near the top.
You have to be running windows os on your computer to use rsdlite. Once all these are downloaded to your computer open rsdlite, power on your phone into the bootloader, I believe its the power button and both volume buttons on the gb leak.
Find the ... button on rsdlite, press this to find the sbf file you downloaded. Once the sbf file is displayed on rsdlite and your phone shows as connected hit start, sit back and flash away.
Running running as fast as it can, running running gingerbread man!