The FIA cage guidelines, and the CARS rules are available online -- I'd start there and then get in touch with your local scrutineer to get the details worked out before you start cutting.
The rules have changed over the last couple of years so going off an existing car will likely get you in hot water.
FWIW, the main cage elements need to be 1.75" x 0.95" DOM or CDS (main hoop, laterals and at least one of the recommended multiple door bars). You need an X in the main hoop and two reinforcing bars in the "ceiling" (can be an X or V).
Extra bits can be 1.5" x 0.95" DOM or CDS.
Lots of extra rules regarding feet, seatbelt mountings and gussets.
It's not rocket surgery but there are many, poorly elucidated, rules...
Or, you can ignore (many of) the rules if you opt for a certified FIA cage kit -- but you must install it exactly as designed, no additions, no deletions or modifications.
Cheers, Ted
Pure mathematics is the enemy of every truly creative man -- Sir Alec