Flanking has been an art ever since the history of woar and Call of Duty isn't any different. However, Ghosts has added a twist to flanking, and by twist I mean it's almost impossible to do...almost.
Ghosts maps are, as you all know, very random. There are shortcuts to get to shortcuts. Alleyways, tunnels, and nooks inside of crannies. The maps are insanely complex and because of this flanking is easy and hard at the same time. The flanker can get incidentally flanked. Compared to previous titles, flanking in Ghosts is one of the most difficult challenges not because the opportunity isn't there, but the maps can turn the tables on you in an instant.
1) Know your maps. It's really hard to flank if you don't know your way around. Learn the maps and discover all the secrets. Each map has a handful of hidden corridors that allow you to cut through heavy traffic areas unseen.
2) Windows make great shortcuts. Nothing makes me laugh harder than an enemy who thinks they have the upper hand when trying to fish me out of a building. It seems no one wants to consider that windows can be climbed through and they're the fastest way to escape. Identify windows that are above doors and you'll find flanking quick and near painless.
3) Set up decoys. Sometimes flanking is nothing more than trickery. Shoot off a couple of unsuppressed rounds to attract attention then hit them from behind. The most basic form of flanking.
4) Keep you back to the edge of the map. As I said, Ghosts is very complex and it's easy for the flanker to be the flanked. If you keep you back to the edge of the map you can't be attacked from that direction.
5) Patience. Sometimes all you need to do is lay low for a few and let the enemy pass. Whiteout is a good example of a map where you can just lay down as the enemy moves up. Once the last enemy passes you take them out.
6) Know each maps vantage points. I feel like a lot of players forget about sniper friendly vantage points. Nothing unravels a flanking opportunity than getting in a firefight with a sniper. Keep out of their crosshairs so you can move quickly.
7) CaC layout. I wouldn't suggest flanking unless you have the right weapon and perk setup. Rushing with an LMG and no speed perks can often lead to you being three steps behind and wasting your time. I prefer flanking with a weapon that handles fast and favors accuracy and damage over RoF. MRs are good weapons to flank with because it's two shots to kill. This allows you to get in, get out, and move on.
8) Don't use equipment when engaging. Flanking often means you're on your own and there's no chance for backup. Once you engage and enemy you're all the support you've got. Make your shots count and don't waste time using flashbangs or the like unless it's a room you're breaching. Throwing a flashbang in an open area like the hills of Prisionbreak will just lead you to alerting the enemy some awry and you'll be facing unfavorable odds. Guns only and make quick work.
9) Flank by not flanking. Sometimes the enemy will try to flank you. Pretend that you're going to head off in one direction but then turn around and continue the way your were originally heading. Some call this the "switchback". This is a chance move but you'll find it works fairly well.
10) Objective are easy areas to flank. If there's an objective such as a DOM point you can always bet there's an opportunity for a good flank. Players get absorbed in the point itself and fixated on that one position leaving them vulnerable.