These secondary missions will begin to become available for you after you complete the Stage Battle against General Lionwhyte towards the beginning of the game. They show up as little white pins in your map (press 'back' button to view map). More will continue to become available as you progress through the game. Complete any 20 of these to unlock this achievement.
I found the ambush missions to be the easiest because you can pretty much end them just by playing the "Bring it on Home" solo.