The example of how to access leptons is given in examples/Example1.C (for electrons) inside Delphes3 installation directory.

The example of how to access missing ET is given in examples/Example2.C of the Delphes installation directory.

This is more complicated due to the presence of pileup and fake jets. In an idealized scenario, we will need identify “signal” jets reconstructed from non-pileup particles and apply an offset correction to compensate for extra energy from pileup. Here is simple method:

1. Build truth jets using Fastjets and non-pileup particles (status=1)
2. Match reconstructed jets with truth jets using a cone 0.15-0.2 in Eta and Phi. Such matched jets will be used for further analysis. The matching is a simplified method to select signal jets (the jet vertex fraction used in ATLAS is not implemented in Delphes)
3. Apply a simple offset correction to correct jets back to the “non-pileup” scenario.

See the correction procedure here