Using the blue overview or radar map on the right as a guide, proceed to and collect the flags depicted by non-blinking yellow dots. The lucky flag will be one of them.
The achievement will not pop until the game is finished. It is earned by obtaining the lucky flag depicted by a red L in the field of play and by one of the non-blinking yellow dots in the blue overview map on the right. (The blinking dot is the special flag) There is always one and only one lucky flag per round. In order to complete a round, it cannot be missed since all flags, including the yellow flag, must be collected to end the round.
The lucky flag is randomly generated and is in a different position each time. In the above video, it is the second flag collected right after the special flag. Since the achievement is not unlocked until 'Game Over', in the video, to hasten the game's end, the player car suicides. Following the game over screen, this achievement along with four others are unlocked.