This can be achieved with any AI difficulty and damage off
After selecting your car for a race the game will load the service area, from this menu select Difficulty and Assists and select Advanced Settings at the bottom.
No turn all of the settings under Assists to off
Dynamic Racing Line
To make this easier i suggest having the difficulty set to 1 and damage off, this will obviously be easier on the earlier events. Listen to your co-driver, make use of the mini map and dont forget you have 5 flashbacks to use.
Here's a video on the first Dirt Tour event to demonstrate