In short:
1. Unlock your phone using the Hard-SPL. Download the file WhitestoneW-HardSPL_V1_99R1_100HSPL.zip found at
http://whitestone.htc-unlocks.com/hard-spl.php and unzip it into a folder. You will have to turn off your anti-virus to prevent the executable from being deleted. You will also need an active internet connection during the process.
2. Download the .7z file of the ROM you want to install and unzlip it into a folder
3. Connect the phone to the PC using ActiveSync. Make sure the ActiveSync window shows the status as "Connected"
4. Run the executable names Imagio_SoftwareUpgrade.exe or something similar and wait until the ROM is fully installed on your phone.
5. The phone will then restart with the new ROM.