Simply go to online and chose championship (which you'll have to do at least one of for a trophy) or single race. The load times are awfully long both at the start and end of each one but let the timer on each screen run out automatically because the game saves after these, so passing them may void your progress. I chose to open a custom lobby using single race of 3 laps doing short courses on Nevada, Donington, Riviera and Wales. This took about 2-2.5hrs for 18 of the 20 races, then did a championship of 2 races with 3 laps in each race. The trophy popped straight after the 20th race (when I did it on ps4 it took about 22races). Because of the load times and poss save problems this really should have a higher tt ratio, but that's a result of not many people having the game tracked.