Your first opportunity at this achievement is the event at the end of the Gymkhana tutorial. Make sure to build up that 3x multiplier, and do as many tricks as you can, trying to not repeat the same thing too many times. Aim for the smashes, air and drifts as they give you the most points for the time.
Here is an example of what you are trying to do:
Note: Later on in the game you will get more chances at this achievement. If you are struggling with the above method, it is possible to get 600,000+ on L.A. Coliseum.