This is my own guide that I made recently for this achievement.
Before attempting this, you need to find a General Store in New York, Boston, and the Frontier. Once you have found one in each area, find an accounting book either in the Homestead Manor or any of the 9 General Stores. You can get the achievement by sending ONE convoy to the New York, Boston, and the Frontier. You do not need to send 3 seperate convoys. When choosing the people that you send the convoy to, just select one person from each area.
You do not need to wait for the convoy to return, it should pop as soon as you send the convoy.