Provided the phone is rooted and has a custom recovery it is actually quite simple.
Step 1: Download the custom kernel that provides the feature (I recommend ElementalX)
Step 2: Be sure that you have downloaded the correct version for your phone. The wrong version can cause some problems.
Step 3: Move the kernel onto your phone through a computer.
Step 4: Flash the kernel through your custom recovery.
Step 5: Choose the options and mods you want. It also has a few other mods and overclocking.
Step 6: Reboot the phone
Step 7: Reboot one more time.
Step 8: Enjoy!!!