Radar Cross Section Calculation using HFSS or CST

Hi,

I am trying to simulate an object's RCS using HFSS of CST. This is my communication design project, deadline is coming in two days and I'm still desperate.

I know lots of things about Radar Cross Section but I couldn't use any of these programs. In the board there are few topics about my problem but there aren't any answers.

I am looking for a tutorial, which can guide me how to simulate from the beginning to the end.

Actually excitation thing confuses me very much.

Thank you.