In Gilded Vale, the first settlement that you go to after the intro and third area overall unlocked on the world map, there is an inn in the southwest corner of the map. Go inside the inn and talk to the innkeeper/bartender woman behind the bar. You will have an option to ask about hiring help (for 200cp) which will take you to a character creation menu like you used for your main character. Create a new companion and the achievement will unlock.