From the main menu, select rivals. You have three different types, monthly, weekly and daily events to accumulate 300 points. They are hot laps (one lap time trial) or pacesetter (three lap time trials, average time counts) events.
You have one month to get 300 points, which is not too hard. I completed one weekly event and about 8 daily events before the achievement unlocked while checking the result the next day. This was early in week 2 of the season. I usually placed somewhere between gold 2 and silver 3. Just make sure you are not starting in the last few days of the season. This might come naturally if you are going for