You download the rom you want and any updates or kernels for it. After download, move these files to your phones sd card via usb, make sure they are on the root directory of the sd card (not inside any folders). Make sure you have backed up any info you don't want to lose as this will wipe the phone. After all this, you power off your phone, once the power is off you have to boot to recovery by holding the home key and the power key. Once you are in recovery, you scroll down to wipe and then pick both wipe data and wipe dalvik cache. If you are on the stock recovery image, you will get an error saying to wipe dalvik via adb but don't worry, it still works, this is just a visual glitch. Once you have wiped these both, press the back button to get back to the main menu and select flash zip from SD card. From this menu scroll down and select the rom file to start the flash, usually about 5 minutes, after its all done select reboot now. This first boot can take anywhere from 3 minutes to like 15 minutes so just be patient. I'd suggest after you have it booted up, power it back off and go back to recovery and re-wipe data just for good measure, this helps fight off any bugs there might be. Good luck to you, post back your results.