After the Inquisition gets started at the end of the prologue, you will be able to go to the War Room and send scouts to check out new areas to unlock. This unlocks the first area called Hinterlands. On this table, you will start seeing other quests show up on the board. These will have you pick one of three members of the inquisition to send their troops on a mission.
These missions take time to complete. It can be as little as 10 minutes or as long as an hour. Whoever you picked to take on the mission will be busy until the mission is complete meaning you can only have 3 missions going at once. Some of the missions also require power to start. To get power, complete side quests in the game until you have enough power to start one of the missions. When the mission is complete you will need to click on it again to collect your reward.
Just finish one of these missions to unlock the achievement.
There are some missions that complete instantly and/or require not power. So far they have all been for collecting mounts available from pre-order or the deluxe edition of the game. I will update the guide if more instant missions show up, but you should have this achievement long before then.