Complete main storyline quest Life During Wartime.
After an unknown amount of time you'll receive a phone call from Elizabeth Peralez starting up the side quest quest "I Fought The Law".
Complete this side quest where upon you will meet River and help him with a case.
After 24 in-game hours River will call V resulting in the side mission "the hunt" which is the first in the official River Ward questline.
After a few days in-game you will receive another call and the next quest titled "Following the river"
Complete this quest and you'll receive the achievement.