Hi Bob,
 
This shouldn't be taken as an definitive installation guide. There are some assumtions in that you need to know how to navigate around a file system from a command line. If anyone has any corrections, feel free to chime in! Having said that...
 
I do not recommend any pre-installed GNURadio installations. This will ensure you obtain the current stable releases of everything needed.
 
You first have to choose an operating system. I chose the latest stable release of Ubuntu 12.04 LTS. That is available here: http://www.ubuntu.com/download/desktop
 
There is nothing remarkable about installing the OS, it's pretty straight forward...
 
The OP25 wiki specifies a GNURadio version of 3.6 and higher. I chose to install the latest, v3.7 from the 'Installing From Source' method. Knowing what I know now, you can skip that step and go straight to installing from the PyBombs repository. This will install v3.7 of GNURadio and all it's dependencies. This will save you about two hours, as the GNURadio install takes this long to complete. PyBombs takes just as long.
 
From the 'PyBOMBS GNU Radio Build Quick Start' page located here: http://gnuradio.org/redmine/projects/pybombs/wiki/QuickStart
 
git clone git://github.com/pybombs/pybombs
cd pybombs
./pybombs install gnuradio
 
Assuming everything installs without errors, you should have a working copy of GNURadio installed.
 
Now we need to pull in the OP25 source and the max-pybombs branch:
 
git clone git://op25.osmocom.org/op25.git
cd op25
git checkout max-pybombs
 
There are two recipe files(pybombs install scripts), gr-op25.lwr and libitpp.lwr that you need to copy from op25/pybombs/recipes directory into the pybombs/recipes installation directory from above. There will already be a 'libitpp.lwr' dependency file in the pybombs/recipes dir, you can either rename it to something else(change the extension), or overwrite it like I did. Once you've copied the files to the pybombs/recipes directory, you need to go to the pybombs installation dir and execute the following command: ./pybombs install gr-op25
 
If everything installs without issue, you now need to set up the runtime environment variables in order to run the OP25 scripts. You should still be in the /pybombs install location, now execute the following command: ./pybombs env
 
This will print out some paths to various things and a location to a shell script that will set these same variables. Execute this script as follows: source /foo/bar/setup_env.sh (use the path that actually prints out on your screen!)
 
That's it, good luck!

-Scott