For this you'll need to be holding 1,000,000 at one time, not accumulative!
I highly recommend fishing as it's probably the quickest way so make sure to complete all "The Fisherman's Gambit" quests to make this a lot easier
First you will need to buy a stack of 99 Earthworms at Seafront then head to the water to the East of the Village (where you'd go to reach the Desert). Once you're here just fish, you'll mostly get Royal Fish which sell for 4000 (good money eh?) In a matter of minutes you're ready to go buy some more Earthworms to do it again, it's just a shame that once you earn 1,000,000 there's not much you can spend it on...