First, make sure Zune is not running on your computer, and get this file to downgrade your SPL to the needed level. To do that, turn the phone off, hold the volume down button, hit power while still holding volume down, then let go of power. When you see the 3 color screen, plug the phone into the computer and run the tool.
Then, get this file. READ THIS POST! You will also need to install the Windows Mobile Device Center software (32-Bit)/(64-Bit) from Microsoft. Following the above steps on your phone, first run the Loader file to install RSPL. Then run the Installer file (again, with the above steps) to install HSPL.
After you do all that, the phone will boot normally and act normal, like you didn't do anything to it. Fear not, it's ready to load any ROM that is listed as a GOLD_C ROM. DO NOT INSTALL ANY ROM THAT DOES NOT SAY GOLD_C OR ARRIVE.
To install the your ROM of choice, follow the steps on the phone and run the ROMUpdateUtility.exe file supplied by the ROM.
NOTE: There is no way to back up your apps or data on the phone when you install a new ROM. Make sure your contacts and stuff are backed up to Windows Live/Google/Yahoo before you do all of this. Make a note of your installed apps, and be ready to lose all your save games.
Again, I am not a developer, and I take no responsibility if something screws up and bricks your phone. I can't stress enough that you CAN brick your phone if you do not carefully follow instructions. Do your research and know what you're getting into.