By MBPr I take it you have a retina model, yes? All you need is an .iso, which you'll get as part of whatever download of Windows 8 you might purchase. Be sure you get x64 rather than x86 so you can utilize all of your RAM. In OSX, all you have to do is go to Applications -> Utilities -> Boot Camp Assistant, instruct it to make a bootable USB flash drive with the .iso you downloaded (it's the first option in the pane if I remember correctly), and then have BCA allocate some free disk space for a Windows partition (one of the two remaining options there). None of the tools for creating a bootable drive linked here are necessary; all you need is the .iso and Boot Camp Assistant. Also, have boot camp assistant download the support files to the root of the flash drive after you've formatted it with the .iso. After the flash drive has been set up and the drive partitioned, reboot while holding down the option key; if you don't see the flash drive showing up, reinsert it at the device selection menu; select the option that isn't EFI boot (I think it's labeled boot camp, but I'm not positive), and in a few seconds you'll be in the Windows 8 installer. Don't forget to have your license key handy. Do you need any further direction beyond there?
