What is the correct approach to obtain PES using CASSCF?
Yes in principle that is a correct approach, but you should be aware of a few pitfalls here and there.

The basis set you chose for your optimisation might not be large enough. I assume you are not going to use STO-3G for your CASSCF calculation. It is better if you run your initial optimisation with the same basis set that you are using for the CASSCF calculation. You will most likely be closer to the minimum and will safe a few cycles when optimising the molecular structure on the larger level of theory.
Picking the right active space might not be trivial. You may have to run a couple of single points to find the right balance of what you must include and what you may neglect.
The SCAN produces a rigid scan of the molecule, i.e. the backbone of the structure does not react with the induced change. That might not lead to the correct behaviour. More sophisticated (and more costly) is a relaxed scan via the ModRedundant keyword.  

Be aware, that CASSCF calculations are quite resource demanding and you probably won't get results overnight.