You'll need to reach level 225 First Aid first, and head out to the respective place listed below.

Alliance

1. Start with Nissa Firestone in Ironforge. She's on the second floor of the herbalist shop in the Great Forge. She will give you Alliance Trauma
2. Head out to Theramore and speak to Doctor Gustaf VanHowzen in Foothold Citadel. You will need to accept his quest and bandage the troops!
3. The worse shape the troops are in, the faster you will lose them. So, zoom out your camera and start bandaging! while one bandage is casting target the next injured troop, and be ready to click. Give it a few tries and you will get the hang of it fast.

Horde

1. Start with Arnok in Orgrimmar. He's in the Valley of the Spirits and will begin Horde Trauma
2. Head out to Hammerfall in Arathi Highlands and speak to Doctor Gregory Victor. You will need to accept his quest and bandage the troops!
3. The worse shape the troops are in, the faster you will lose them. So, zoom out your camera and start bandaging! while one bandage is casting target the next injured troop, and be ready to click. Give it a few tries and you will get the hang of it fast.