Unlock first Perk
Story related and cannot be missed. Note that unlocking a perk will not automatically equip it; you must equip perks manually. Unlocking perks is scored based; once the target score is reached, a tier of perks will automatically unlock. By pressing to pause the game, you can go into Player Perks. Press to enter the menu, then select a perk and press to equip it.
TIER 1 allows you to equip 1 perk.
TIER 2 allows you to equip 2 perks.
TIER 3 allows you to equip 3 perks.
TIER 4 allows you access to the best perks in the game. However, your equip limit remains 3 perks.
If you are curious how close you are to unlocking the next perk tier, check the mission summary page (this can also be done from the level select under Main Menu, Campaign, for any levels you've completed). On the bottom left hand side of the screen, look for your score. To the right of your score will be a "/" followed by a blue number. This blue number is the score required to unlock the next perk tier.