From the main menu choose Multiplayer then Online Multiplayer. Then you can choose either Quick Match, Player Match or Create Private Game. I'm not exactly sure if you can get these in a private game or not but if you hit Quick Match and can't find a match, you are prompted to create your own game. Choose "Yes" then set your special stage to the Wales Rally GB - Alwen which is the last special stage. The track is only 2.17 km so it's very short and quick. Confirm your options and ready up. You can either wait for a random to join (which is unlikely as this game is all but dead online) or you can invite your boosting partner to the race and start it once you both ready up by hitting X in the lobby. Now simply race the stage and finish in first to unlock the achievement.