To do this you must capture the 15 outposts in the game. This does not include bonfires, just outposts. So you are looking for the red symbols on your map with the triangular shape.
The first few you are likely to do are easy ones and won't cause you any hassle as the enemies will be weak. However once you get on to ones which are labelled 'Hard' and 'Very Hard' you need to be strong enough to take it on. By the time you get to this point in the game you will know what your limitations are on mission toughness and whether you are strong enough to take it on.
You can also opt for the stealth approach. Best way is to use your owl to scout the area and tag the enemies, maybe take a couple out. Now you can either go in yourself and take them out one by one or send your beast in. If you send your beast in they won't know you're about, however your beast is likely to die and you may go through most of your tamed beasts with this method. I preferred to go in myself and take them out. Just be weary that if you get caught and you haven't destroyed the enemy horns they will call for reinforcements. Not the end of the world you can call in your beast to help and use all your fire-power but you will have a struggle on your hands to beast them.
If you solely go for outposts you can have them done in an hour or less. If you can't find any more on your map but you still have some left to do, just look for a red area on your map or maybe a bit which is cut-off. I.E. you haven't explored there yet. Just go to these areas and you will likely find an outpost or bonfire.