Hey freezy!
First just be sure you have the rom on the sd card...I know you said you did, but just be sure

Second, boot into recovery....(push and hold the volume up, then the home button, then the power button.) Continue holding all three buttons until the phone vibrates. Then let go of the power button while continuing to hold down the volume up/home buttons until the screen comes on with your recovery.
Take note of what recovery you have because I am curious
Anyways, whilst in recovery, there is an option to backup....a nandroid backup....please do that first. That backup is a snapshot of your phone's current state. Kind of like a restore point. It's good to have
Then, whilst in recovery, choose to wipe cache, dalvik cache and factory reset (or system wipe or data wipe) then navigate to your sd card to find the rom and flash it.
If you are not clear on something, just ask