Warning, this could brick/bootloop your phone, only continue if you can recover form these errors.

Requirements:

Phone is rooted
CWM is installed
Battery is at least 50% or phone plugged in

1. Copy the ROM file over into your phones memory or SD card.

2. Boot into your recovery (Volume up, power and home button on Samsung) not too sure on other phones but find how to boot your phone into recovery or use the ROM manager app and reboot into recovery using that.

3.Once in CWM Recovery, choose wipe data/factory reset. This will erase all of your settings, apps, app data, and everything except for your personal photos, videos, and files in your internal storage. (Does not touch external storage.)

4. Go back to the main menu and select "install zip"

5. Choose choose zip from /sdcard or choose zip from /external_sd depending on where you put your ROM zip files. (/sdcard = internal storage, /external_sd = microSD card)

6. Find the ROM file and select install and wait for the process to complete.

7. Once everything you need is installed go back to the main menu and click "reboot system"

8. The phone should boot up with the new ROM installed!

If you have any problems please PM me, I will be glad to help!