For video guides, I recommend using THIS THREAD.
There are 35 total dog tags to be found. If you're playing co-op, you must pickup the dog tags for them to count. If you don't see a dog tag at the correct location, try pressing around the area to pick it up. For some reason, they sometimes don't appear on screen even though they are there. Once you've picked it up, it is saved to your service record. To check your progress from the main menu, press on "Service Record" and again on "Special Projects".
The breakdown for each level is:
Level 1- 3 total (Hicks, Moore, Trubetskoy)
Level 2- 3 total (Dietrich, Forcher, Ziglar)
Level 3- 3 total (Frost, Greenlief, Ashley)
Level 4- 4 total (Norton, Wierzbowski, Vasquez, Macaulay)
Level 5- 3 total (Gorman, Bradley, Jelley)
Level 6- 3 total (Bailey, Washenfielder, Spunkmeyer)
Level 7- 3 total (Apone, Barrington, Nelson)
Level 8- 3 total (Crowe, Capes, Baroski)
Level 9- 4 total (Drake, Watkins, Rechner, Siwiak)
Level 10- 3 total (Ma, Morales, Ferro)
Level 11- 3 total (Papp, Montgomery, Hudson)