You mean install Windows? Bootcamp is a part of the MacOS, no need to install.
If yes, then all you need is just boot from the Windows disc (highly recommend use a disc for cMP Windows installation), or USB drive. Then install Windows.
Bootcamp assistant only do 4 things.
1) partition your hard drive, you can do the it in disk utility with less restriction. Anyway, for cMP, highly recommend install Windows on a stand alone hard drive. We have plenty of internal HDD bay, no point to install Windows on a partition which may eventually screw up the MacOS (if something goes wrong). So, this part can be completely skipped.
2) download the bootcamp driver. You can go to Apple website and download it by yourself.
3) launch the Windows installation disc after reboot. Obviously you can do this by yourself.
4) run bootcamp driver installer after Windows installation. Also, you can run the installer by yourself. And this is actually optional.
