Securing Draenor

Completing scouting missives counts for this achievement. You can also work on achievements while on scouting missives.

Here's where to find these broken down by zone and with general coordinates that will get you in the area to complete the assaults.

As noted in many comments above, make sure to buy the scouting missives or have the corresponding daily quest!

Frostfire Ridge
Assault on the Iron Siegeworks (82, 56)
Assault on Magnarok (70, 26)
Assault on Stonefury Cliffs (39, 20)

Gorgrond
Assault on the Everbloom Wilds (57, 45)
Assault on the Pit (45, 26)

Nagrand
Assault on the Broken Precipice (37, 25)
Assault on Mok'gol Watchpost (45, 29)

Shadowmoon Valley
Assault on Darktide Roost (58, 90)
Assault on Socrethar's Rise (49, 75)

Spires of Arak
Assault on Pillars of Fate (74, 33)
Assault on Skettis (Fly to Akeeta's Hovel (65,17) in Shadowmoon Valley. Follow the trail north. Skettis starts at 60.67, 10.44. - comment compliments of Dalylah)

Talador
Assault on Shattrath Harbor (40, 23)

WARNING: Unless you are actually on the daily quest or have purchased a missive, you won't get credit for completing these objectives. I tried to use this as an opportunity to catch up on my garrison campaign quests. You'd figure that, hey, if Blizzard is sending you out to Socrethar's Rise and the objectives are active and you can complete them and you get the text "You have completed Socrethar's Rise," you'd actually get credit for this.

Not so.

3+ hours down the drain as I completed numerous areas and didn't get credit for any of them.

TL;DR: If you haven't picked up the daily or purchased the corresponding missive, you're wasting your time.