Step 1: Find and fight a Warchief (Must have "Command" unlocked from Main Mission progress)
Step 2: Weaken him until he "surrenders". (Do not use executions when he is in low health, he must stay alive!)
Step 3: When he surrenders, choose "Dominate" then "Command"
Step 4: In the Sauron Army screen, choose another Warchief (unbranded) and "Riot" to attack him.