To unlock the Bountiful trophy in Red Dead Redemption 2, you must commit enough crimes in each state to earn a $250 bounty in each, causing bounty hunters to actively pursue you. Once you have earned the $250 in the final state, you must survive three consecutive days on the run from the hunters.
Ambarino
Lemoyne
West Elizabeth
New Hanover
New Austin
I would recommend leaving this until the epilogue so that you have a good chance of survival and also to allow you to have finished off the below trophy, which requires high honour:
If you care about your honour levels at this point, make a manual save. Earning $250 bounty in each state is a lot of crime.The easiest way to earn $250 is to take out a lot of law enforcement. Although this is pretty straight forward in every area with a major town, Ambarino doesn't really have one. The easiest way to sort this out in Ambarino is to wait for a train to come along (or hop on one elsewhere and allow it to travel through) and then rob it. Law enforcement will come after you, and there is plenty of cover on a train to take them all out while you rack up a bounty. You can even blow up the train engine with dynamite to get a rapid bump (I immediately hit $900 when blowing up a train in Ambarino).
Note: You don't have to survive from start to finish with this; if you get killed with e.g. 3/5 states at $250, just carry on with the next state when you resurrect. All that matters is that you survive three consecutive days once all states have a $250 bounty.
Once you have your $250 in each state, you can easily survive three days by camping out in the wilderness for the maximum allowed time, leaving the camp and setting up another. You'll find that you can't immediately go back to sleep in the same camp each time because normally bounty hunters will have caught up with you somewhat -- the game won't let you sleep or make a new camp while there is an active pursuit. You can either deal with them or, even easier, just ride off in the opposite direction to their red markers on your mini map until you are able to set up a new camp. Three or four sleeps should do it. I recommend New Austin for this camping trip as there's not a lot of law enforcement or innocent bystanders to make things more difficult.