First you have to buy the 1987 Porsche 959 for free. Then you can start a new championship with 4 different events in Supercars / Retro Supercars group or you can complete 4 events in multiplayer. If you want to complete this fast, go to the Saint-Martin car meet (near to the airport) and play showdown events. The Bucket List events and the Head-to-Heads against Drivatars doesn't count, but you can play championship or online events. You don't need to win, just drive to the finish.
After this achievement you need only 6 more races in this car to unlock this achievement:
You can easily complete other Porsche Expansion achievements if you read my solution here: