How to install the development version of atk9k
At the time of writing, ath9k will not compile against the current kernel release. Thus, in order to get it running, you will first have to download and compile the wireless testing branch of the current Linux kernel. Once you have that running, you can then install the ath9k driver itself. This how to will outline the entire process without assuming any prior knowledge of git or kernel compilation.
You will need to have git up and running to access both the wireless testing branch, and the ath9k driver itself. Git is the revision control system used for the Linux kernel. Your distribution will most likely have a package for it so install that using aptitude, yum or portage or whatever first.
Get the wireless-testing branch
Once you've got git installed, you can follow the linux wireless git-guide and download the wireless testing branch using git-clone.
$ cd /usr/src
$ git-clone git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless-testing.git
Configure the kernel
This is the part most people are afraid of. However, you use your current kernel's configuration as a baseline to help ease the pain. Most self-respecting distributions keep a copy of your kernel configuration in the /boot directory alongside the kernel itself. If this is not the case, you might find a copy of the kernel config at /proc/config.gz.
$ cp /boot/config-`uname -r` /usr/src/wireless-testing/.config