As a Rough Guide I would say this:
Start with the slower cars.
Focus on keeping the car on the road.
Be in the same gear as the corner grade (left 2 would be 2nd gear).
Once you get comfortable finishing without heavy damage, I would look to start shortening the racing line through cutting corners more and holding more speed and maybe the next gear up from the corner grade.
Presuming you don't have a wheel, I would keep the assists on and once you're more comfortable, try disabling traction control and/or stability control.
ABS makes a huge difference so I would always keep that on until you are ready to modulate your brake pressure, as that's pretty hard to do, especially on a controller.