The easiest way I know of is to use greenthumb’s sevenblend script. After installing the necessary dependencies you simply run the shell script and it downloads and builds the latest svn versions of both Blender and Yaf(a)ray. To update them later you need only to run the script again.
I would be happy to help you with it. I am still using 8.04 so I don’t know if the dependencies are the same(there is a list included with the script in the README), but it really is quite straightforward. If you have built Blender before it should be easy. It builds an integrated version of Blender using the old Yafray button to render so you can still use particle hair and save animations as .exr or whatever, or you can choose to use the QT python export from the same Blender build.