...
If you don't already have it (python external libraries under GLAST_EXT don't count) install python from the official site . Version should be at least 2.5; 2.6 or 2.7 preferred. Do not install anything attempt to use a version in the 3.x series.
SCons
You'll need 1.3.0 or newer; 2.1.0 is preferred. You can get it from the official SCons site .
...
First determine what specific tag you want. (Currently - As of May 26th - LATEST 5670 looks like a good bet.30th - LATEST 5677 is preferred as soon as it's available. Or get 5676 and do cvs update -A SConsFiles) It can be checked out of CVS
cvs co -d GR-5670 5677 -r GlastRelease-LATEST-1-5670 5677 GlastRelease-scons
(note the -d option above is not necessary. Including -d GR-56705677 will cause the top-level directory of the check-out code to be called GR-56705677. By default it would be named after the module being checked out; i.e., GlastRelease-scons)
...
/nfs/farm/g/glast/u52/ReleaseManagerBuild/Windows-i386-32bit-vc90/Debug/GlastRelease/GlastRelease-LATEST-1-56705677-source.zip
Replace "source" above with "devel" to get a zip file which includes final build products along with source.
...
For any RM tag you can find out what externals were used by examining the compile output for that build. This output can be found for each build in the RM II web pages or by running the stand-alone program RMViewer , available via ftp . The list of externals used is near the top, following a string of lines starting "Checking for C++ ..."
Building Preliminary Targets
...
scons --with-GLAST-EXT=%GLAST_EXT% --compile-debug --vc9 to_install
or use GoGui , available via ftp, to do the same thing.
The first creates a file, _setup.bat, which defines environment variables needed before starting up Visual Studio or for running executables belonging to GR once they are built.
...
to_install installs all headers, xml files, job options files, etc. (Recall that, unlike CMT, SCons as we have customized it installs all source files needed at build or run-time as well as libraries and executables.) Almost all of these installs can now be done inside Visual Studio but some critical ones are missing. I hope to fix this before long As of GlastRelease LATEST 5677 (in particular, SConsFiles-00-23-05), the Visual Studio project files generated will take care of installing all headers and other source files, but not quite always in the right order. To be certain to avoid compile failures due to missing headers, it's recommended you start out by installing all source files from SCons. Re-installs after updates can be done from within VS.
Starting up VS
Bring up a VS 2008 console window. Run the setup file created above. Relative to the root directory of your source installation, it will have a path more or less like
...
Then start up VS on one of the solution files, e.g.
devenv allGleam.sln
Alternatively you can start up a solution file from within GoGui. Select the root directory. Then, in the bottom navigator pane, navigate to the subdirectory with a name something like studio\Window-i386-32bit-vc90-Debug. You can start up VS by right-clicking on any solution file and select the Studio 
menu item.