A Champion Star is the reward for having a driver reach Level 50 in Public Multiplayer.
To do this you will need to take part in Public Multiplayer races, and earn XP after each one.
You earn XP for doing various things, such as:
> Finishing a race.
> Finishing in the top half of the field.
> Winning a race.
> Finishing on the Podium.
> Having a penalty-free race.
You can also earn XP for completing various Milestones, such as:
> Reaching a certain number of races completed.
> Reaching a certain number of races won.
> Reaching a certain number of kilometers driven.
XP cannot be earned in the Online Championships or Custom Game portions of Multiplayer, which means there are no ways to shortcut this Achievement. You'll just have to grind it out and if you're fast/lucky enough you'll get more XP on average per race as the skill gaps between players seems quite high so far.
You can let the AI drive for you by pausing during a race, but your XP earnings will be drastically reduced.
If you Retire from a race your XP earnings will be drastically reduced.
Depending on your performance these are the approximate amounts of XP you can expect to get per event type:
> 5000 XP for a 5 Lap Race
> 12,000 XP for a 25% Race
> 25,000 XP for a 50% Race
> 1500 XP for Qualifying
During my time in Public Multiplayer these were my typical XP earning rates:
> 16000 XP per hour with only 5 Lap Races
> 25000 XP per hour with only 25% Races
> 40000 XP per hour with only 50% Races
The amount of XP required per Rank increases with each successive Rank, to the point where you will need around 33,500 XP to go from Rank 49 to Rank 50. I have not been tracking XP totals from the beginning but if I were to take a guess I'd day you would need around 1,100,000 XP to go from Rank 1 to Rank 50. I would advise you to not look at the big number and quite literally take it 1 Rank at a time.
Some other notes:
> Very few people do 50% races. Most of my races only had 2-3 people in them. This gives you a gauaranteed podium finish and if you avoid damage and penalties you could earn up to 35000 XP per race.
> 25% races typically have 5-10 people in them. They are a little riskier than 50% races but if you crash and retire you won't lose 40 minutes of XP earnings.
> You get the same amount of XP for running a short variation of a circuit as you do a long one, so if you're trying to optimise your time vote for short circuits wherever you can.
> Assists make no difference to the amount of XP you earn.
Along the way you will also unlock the following:
Good luck, you're going to need it as this Achievement requires over 50 hours of high quality driving against random online opponents.