How to install Windows 8 on Macbook Pro w/ Retina Display?

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As you said, OS X 10.8.3 includes Windows 8 compatibility, so you can install it through Boot Camp.

It's as simple as open Boot Camp Assistant (it's stored on Applications > Utilities folder) and follow the steps to install Windows 8. If you want all the steps, Apple has just released the steps to install Windows 8 on Mountain Lion > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.8.pdf

1. You are referring to Windows 8 Pro. As you have a MacBook Pro with Retina display, if you bought Windows 8 on a disc, you have to buy an external optical drive and use it to install Windows. If you purchased it on the Microsoft Store as a download, you got it on a ISO file, so you can create a bootable USB drive with the Windows installer using Boot Camp Assistant. To install Windows, you don't need anything more than the disc (or the ISO file), a USB drive (to install Boot Camp drivers) and the product key.

2. You may notice problems with them just after installing Windows, but they will work properly after installing the drivers

I don't recommend you to use a 1 TB HDD. Boot Camp will format the whole HDD in order to store the Windows support software, so it's better to buy a small USB drive. They are cheap and any will work properly for you. Just make sure that you buy a 1 GB or bigger drive

Just as you would do with a DVD, but you have to tick the first option in Boot Camp Assistant (the one to create an install disk). In this case, it's easy, because Boot Camp Assistant will search automatically the USB drive and the ISO file.

There's one thing I don't have clear enough: are you going to get Windows 8 from the Microsoft Store as a download or you already have Windows 8 on a DVD? In the first case, you need a 4 GB or bigger USB drive. In the second case, a 1 GB USB drive is enough to store the Windows support software

As you have already purchased it, just go to the Microsoft Store, log in with your Outlook account and redownload it for free (you should see a link to your purchases at the bottom of the site). Also, in case you need it, you can access also to your product keys

Correct

Wait until that finishes. If you are installing Windows, the installation may take half an hour or more. If you are copying the Windows support software onto the USB drive, this may take some minutes

You may notice that the bar doesn't move. Depending on the USB drive you have, this may take more or less. Be patient. If, after 30 minutes, the bar hasn't moved, something is happening