[Samsung Galaxy Nexus] Cloudy_cloud:
Hope this helps.

Hey man, we all started out somewhere. Not sure your level of comfort with custom ROMs, but your best bet *read easiest* is to download an app called 'ROM Manager' after you've rooted your phone. I am assuming you have rooted and unlocked your boot-loader (you will see a little unlock image when you turn on your phone). This will allow you install the latest ClockworkMod Recovery (referred to as CWM a lot). This is basically a separate boot program which you will then use to flash the zip files (the ROM zip, GAPPs zip, and if you want a custom Kernel zip) to your device. ROM manager also lets you backup your current ROM **DO THIS BEFORE INSTALLING A CUSTOM ROM** in case something goes wrong. I find CWM easier to use that TWRP - the other program used to "flash" (install) zip files. The non-touch CWM version (which you can get for free from ROM Manager) uses the up and down volume keys to move up and down the menu, and the power button to select an option.

ROMs are device and provider specific. As said earlier - Verizon Gnex is named toro. GAPPs and Kernels are device specific, but not (as far as I know) provider specific. I would definitely recommend doing a full wipe (called something like factory reset/wipe all data) in CWM the first ROM you flash. Really, it is safest to do this between any different ROM (known as clean install). If you are upgrading a ROM to a newer version and the base android is the same (e.g. Purity 4.3 to a newer version of Purity 4.3) than this is not necessary (dirty install). However, an older version of Purity (4.2.2) to Purity (4.3) this would be necessary. This is the order I would do things the first time...

ROM Manager:

Flash latest CWM

Backup Current ROM

Reboot Phone > Hold down and up arrow > move into 'Recovery' (into CWM)

CWM

You'll see a blue screen with options.

Go to Install File and make sure that your Purity ROM zip, GAPPs zip are in there..

Go back to main page of CWM

Factory Reset/Full Wipe (this does not effect any personal files like pictures or the files of the ROM/Gapps you put on your phone) and you can skip this step if you are just upgrading a ROM like mentioned above.

Wipe Cache Partition

Go to Advanced > Wipe Dalvik Cache

Back to Main Page > Install Zip > Choose the ROM zip (this will take a min to load)

Install the GAPPs file now

I always wipe dalvik Cache again (not sure this is nessary but I've never had a problem)

Flash any other needed files by the ROM Original Poster (looks like SU zip file in the case of Purity)

Reboot Phone

Note that the first bootup can take up to 10 minutes (although mine has never gone past 2 min).

EDIT: I never flash a custom kernel at the same time as a new ROM/GAPPs although you should be able to. I wait a few hours after a new ROM install and then go back into CWM to flash the Kernel. To flash a kernel that way all you have to do is wipe dalvik Cache, install Kernel zip, wipe dalvik Cache and then reboot. Just make sure that the Kernel base version (4.2.2.or 4.3) matches the ROM you have.

EDIT 2: Please correct me if I am wrong or add details if needed. I wrote this rather fast.

Once you get the hang ROMs, it's not too hard or scary, but can be intimidating at first. Hope this post helps anyone new as I could have used something like this when I started.