Watching the tutorial you just posted, you can see the user creates two boxes: the first, smaller box is pec (perfect conductor) which is what he/she wants to evaluate the RCS of. The second box is how they begin going about making their boundary conditions, in this case a PML (perfectly matched layer, which simulates free space). HFSS has a helpful way for making the PML for you, which is displayed in the tutorial, and mentioned a bit more in detail in this thread:
https://www.edaboard.com/threads/3146/
The user in this tutorial also defines some mesh operations; this is not something you need to get into right now, that's typically just a way to speed up sim time.
After this the user defines their solution setup: note they change the solution frequency to the frequency of interest, for you this is the frequency the radar operates at!
Finally the user runs the simulation, waits a few minutes, and then goes under "radiation" and creates two far field setups. From there they can go to the fields report option and use the RCS command to calculate the RCS. You can go on to plot this value as well.
For your application, you can follow this video the same way, except for instead of the inner cube generated at the beginning, you should place your wing, and instead of assigning a pec material, you can use something more plane like, such as aluminum. Or perhaps you would have a better idea of what material to use, I'm no aircraft expert. Again, unless your wing is a very simple geometry, I recommend looking into importing geometries into HFSS from other CAD software.