The generator fragment takes in the LHE file generated by the matrix element generator(eg. MadGraph), and performs processes like hadronization on the sample.
This hadronized sample can then be processed by the CMSSW to do things like Fast Simulation.
At the end I'm giving the cmsDriver
command to perform the complete Fast Simulation in the way we have done, if you already
have experience with CMSSW you should be able to modify this to suit your needs.