1 Sneak behind the enemy
Sneak behind the orc you want to brand.
2 Rotate the screen
Rotate the screen until the orc is directly in your sight.
3 Wait until the orc glows red
When the orc glows red this means that he is in your range of branding and killing.
4 Press the branding button
When the Orc glows red the possible options will appear on the screen. Press on the button that says brand and that Orc will fight for you.
5 Activate branded orcs
After activating orcs you can then give them the command to fight for you by clicking the up arrow (PS4).
6 Captains need to be weakened
To brand a captain you need to weaken him first. Attack the captain until his health goes down then grab him. After grabbing the captain you will be given the option to brand him. (See How to interrogate enemies in Shadow of Mordor?)
7 Isolate captains first
It will be hard to brand a captain in the presence of other orcs as they might attack you while you are grabbing him. Try to isolate the captain first.
8 Branding needs to be learned
The branding ability can't be done without learning it. This will happen later in the game, specifically after defeating the black captain and moving to the new area. (See How to Kill Warchiefs in Middle-Earth: Shadow of Mordor)