You get chaos from making military owned assets explode. Everything from the items you must blow up to liberate settlements to the random enemy jeeps driving around counts.
In the course of obtaining the 100% achievement
, you should earn almost exactly as much chaos as you will need to get this achievement at the same time. I reached 100% game completion with 1,060,000 chaos.
The only real way to grind this if you end up short at the end of the game is to revisit re-oppressed areas, whether that's by replaying weapon frenzy events or using the "Re-oppress and fast travel" option on a settlement after campaign completion. The chaos you generate is always added entirely to your total chaos, and not simply e.g. the extra 5,000 you got over your personal best score.
Chaos also accumulates during the DLCs, so you have more opportunity to work on this should you have not accumulated a million chaos in the retail campaign.