Collecting Dogtags:
To get a dogtag, simply go behind a guard (about 5 steps away) switch to first peron view and point your gun at the victim Youll hear your character say freeze. When your enemy is in the hands over head position, walk round to the front of them, and point the gun to their head (still about a step away) theyll give you their dogtags.
The rewards for tags are as follows:
Tanker:
30-40 tags: Bandana (Infinite ammo)
60-70 tags: Stealth (invisibility)
Plant:
120 tags: stealth
80 tags: brown wig (same as bandana)
160 tags orange wig (inf. grip)
361 tags in the whole game: Blue wig (infinite air)