Use a weapon or grenade that can trigger repeated damage events
Find a vending machine or target that can be hit repeatedly
Equip gear that boosts money drops or loot chance if available
Fire or throw in a way that causes multiple hits on the same target
Pick up the money that drops each time
Repeat the process as long as the target or setup allows
Save and quit if you need to reset the farm
Use a co-op partner to speed up repeated kills or resets