Total War: THREE KINGDOMS

"Share the spoils" is an army stance unique to bandit factions. When you select your army, there's an icon above the first general to the left (the commanding general). Hovering over that icon brings up the list of all the stances, and Share the spoils is the last one on the list. Loot replaces supplies for bandits, and you can see how much your army has by hovering the red bar with the sack icon in the bottom left corner.

You can do this with any of the bandit factions (Zheng Jian and Zhang Yan in vanilla, and Yan Baihu through free DLC), but you should be able to get it on the second turn with either Zheng Jian or Zhang Yan. Fight your first battle and make sure to pick the loot for your rewards. Then immediately end your turn. When it's your turn again, make sure your loot is full (the bar should be fully dark red/maroon-ish and you should have 100/100 loot), then select the Share the spoils stance. The achievement should immediately unlock, so you can turn the stance back off and play your turn out normally or quit the game (should not have to end the turn).

If you want to do this in a normal game, you can set your army stance to raiding (3rd or 4th on the list) to generate a passive amount of loot (25+ based on the territory you're in), in addition to the amount gained from fighting battles. Edit: should probably add that looting with raiding stance only works when in enemy territory.

Edit: changed wording to try to make it more clear that Zheng Jiang and Zhang Yan are in the base game and only Yan Baihu is free DLC