OK - for this achievement you just need to have collected over one million coins in total across the whole game. This can be on race tracks, stunt parks or as winnings.
On race tracks & stunt parks the small gold coins are worth ten each & the larger blue starred gold coins (which replace the car part crates, once collected) are worth 100 each.
As a bonus in the stunt parks the smaller coins will come in lines of five. Collect all of these & another row will spawn nearby. Collect all the rows in a sequence (it could be anywhere between two & ten groups of five - maybe more) & you get a 250 coin bonus.
The quickest (but poss'ly most boring) way to get the million once you've finished with career is to grind the 300 HP Grand Prix in Championship Series.
You get 130 thousand for a win.
The one million does not have to all be in the bank @ the same time so feel free to continue to buy cars while collecting.