# 1) Check the geometry differential code out of CVS and setup the scripts   
$ cvs co StarVMC/StarBASE   
$ ln -s StarVMC/StarBASE/macros/starbase.py .   
$ ln -s StarVMC/StarBASE/macros/differential.py .   

# 2) Check out the MC geometry  
$ cvs co pams/geometry   

# 3) Compile it all under DEV   
$ stardev   
$ cons     

# 4) Now run the starbase application on the geometry which you will   
#    be modifying.  Be sure to select an eta and phi range which is   
#    appropriate for your detector. The following example treats the   
#    case where we want to modify the EEMC (ECAL geometry).  Note that   
#    phimin and phimax are in degrees.  The code takes some time to  
#    execute, so we limit it to a 30 degree sector.  [1]  
$ starbase.py --name=ecal_baseline --geom=y2006h --etamin=0.8 --etamax=2.2 --phimin=-15.0 --phimax=+15.0    

# 5) Make your modifications to the geometry.  When you are done, you  
#    can compare the material budget vs eta by running the differential  
#    code  
$ differential.py --base=ecal_baseline --geom=y2006h --top=CAVE   # CAVE and daughter volumes  
$ differential.py --base=ecal_baseline --geom=y2006h --top=ECAL   # ECAL and daughter volumes  
$ differential.py --base=ecal_baseline --geom=y2006h --top=EMSS   # etc...

Notes:

[1] Do not use dashes ("-") in names.  ROOT will not be able to create the geometry with a "-" in the name.