Recently i have tried to install Windows 7 in a Mac Mini and a MacBook. I have a Window 7 installer in a bootable usb. Just keep pressing the option key on startup and you could select the boot device in the boot menu. It works fine for the Mac Mini and the Windows 7 is installed without any problem.
But after the installation, i found that many drivers are missing. Finally i found a blog post written by Chris F Carroll. You can find all the details about getting the drivers from the Boot Camp package in that post so I am not going to repeat it here.