Dear Sujan Sarkar,

For CASSCF calculation example, you could find an example and a exercise in the chapter 9 of the book "Exploring Chemistry with Electronic Structure Methods", 2nd edition by James B. Foresman and Aeleen Frisch. If you don't have access to this book I will be pleased to send you a pdf of this latter.

Honestly, Gaussian is not a good package for CASSCF or MCSCF calculations in general. I would recommend using Molcas/OpenMolcas (https://www.molcas.org/introduction.html) or Molpro which are well-known for their efficiency as well as large community of the users (https://www.molpro.net/).

Dear Sujan,

This link might be useful if you're using Gaussian:

https://www.researchgate.net/post/How_to_make_an_input_file_for_CASSCF_single_point_energy_calculation_for_a_molecule_at_triplet_state_T1_in_gaussian_09

Thank you everyone for your helpful suggestions!