Unpack the data.acd of the car using CM, then have a play with the data in the aero.ini. For basic aerodynamics, generally I copy the aero.ini and each wing .lut file from a well sorted car that's similar. Then use the CM showroom to position the body/wings. After that you can adjust the main aerodynamic properties simply by adjusting CL_GAIN and CD_GAIN in the aero.ini. CL_GAIN determines downforce or lack thereof, whereas CD_GAIN determines drag.
Download this track:
https://www.racedepartment.com/downloads/test-pad.10703/
It's very simple so loads quickly and if you've enabled Developer Apps, then you can open the Aerodynamic Debug app and run up and down the 9km straight. You'll then see the values generated by each part of the aerodynamics with regard to drag and downforce at various speeds. All you need to do then is fiddle about with the aero.ini until you get your desired result.