Gather all the memory cards.

There's a total of 20 Memory Cards on the islands and all are within the Outposts you liberate. In each new Outpost, you'll have your own building where you can stock up on equipment. If you then look closely on the map, over some Outpost buildings you will notice a Computer Chip icon (Like the one in the achievement tile).

Head inside of your building and inside you'll see a laptop. Simply grab this by holding X and the Memory Chip will be yours. Now keep liberating Outpost to find the rest.