Go up to the shield activators around the map - there are 3. Use them when available at least 3 times in one match. Online Matchmaking only.
You do not have to use each one. You can use one of them 3 times. To activate, hold reload button until the hologram goes away and the shield shows up.
AJ6627 - You don't have to be the one who activates it, only at least at the switch holding the activate button when it activates.