How to install Windows 10 on unsupported Mac?
A different user recently posted a similar question on superuser. Since my answer was accepted as correct, I will repost it below.

The Basic steps to install 64 bit Windows 10 are as follows. I assume here that your current configuration is the default "Out of the Box" OS X installation.


Download the correct Boot Camp Support Software for your Mac.
Transfer these files to a FAT formatted flash drive. For your Mac,
use the same software as for a 64 bit Windows 7 installation. These
files can be found at Boot Camp Support Software 5.1.5621
Use the Disk Utility to reduce the size of your OS X partition. In
the empty space, create a FAT formatted partition with the label
BOOTCAMP.
Burn the Windows 10 iso file to a DVD. You can use the Disk Utility
to do this. I always use a RW DVD. This takes longer to burn, but is
reusable.
Restart the Mac with the DVD in the optical drive. Hold down the
alt/option key and select the DVD icon with the label Windows.
Install Windows to the Boot Camp partition. You will have to tell
the installer to reformat this partition as NTFS. DO NOT CHANGE THE
PARTITIONING DURING THE INSTALLATION PROCESS.
At some point during the installation, the computer will reboot.
When it does, it will boot to OS X. This is to be expected. Go to
System Preferences and instruct OS X to boot back to Windows. The
installation process will continue.
After the Windows installation has completed, install the Boot Camp
Support software stored on the flash drive. To accomplish this, you
will have to right click on the icon and select Compatibility Mode.


I do not consider the process to be that difficult. The most challenging step is creating the FAT formatted partition labeled BOOTCAMP. If something goes wrong, you are in danger of loosing data on your OS X partition.