I was in the Marines from 2000-2004, and I'd say I have a love/hate relationship with my experiences therein. I'm wondering how I could incorporate them into fantasy writing. Could you tell me where I might look for other devil-dogs or branched otherwise veterans who can help me develop my stories?
My usual TLDR: Worry more about getting the "fantasy" right than the "military". Your experience will be of service to you almost without you really knowing it.
In the end you won't be able to help writing from experience, even unconsciously.
It is said that you should write what you know. I've never really done that consciously, but to make "real" characters I have always mined my current experiences of incredibly dull everyday life; which I happen to have a wealth of experience of. So when I was young my protagonists were school kids. When I was in my twenties they were people who were doing "jobs" and regarded their social life as their life. When I got to my thirties they were just people trying to cope with a work/life balance and thinking about families and serious relationships and stuff.
Then I stuck a bunch of angels, werewolves, alchemists, vampires and old gods into their lives. The verisimilitude of the protagonists was always meant to "sell" people on the really wacky stuff I was including along with.
The first novel of mine that's at all readable is about college kids getting involved in a war between vampires and werewolves (and I wrote it in 1995 before that was a proper cliche go me!) re-reading it now the vampire and werewolf stuff is kind of humdrum, ironically, but the college kid stuff still pops because I was unconsciously writing it with a level of emotional and practical detail that was right at my fingertips. The point is that if I were you I would worry about making the story of the Necros bit compelling and the military stuff will just kind of be interesting.
I wouldn't worry about trying to force your military experience out it will come out. I also wouldn't worry overmuch about your "technique". Write your military fantasy as the story you think it should be. I don't think there is an ABC for all military fantasy and if there is it should be read, digested and then used as a buffet, concentrate on the bits you like and ignore the bits you don't. The thing no guide can cater to is that you have experience and that is what will come out so if you think part of some "one size, fits all" plan is junk, then junk it will be for you.
Any technique you encounter in written form will be a lesson for military and non-military alike. If I wanted to write a military fantasy then I might come to you to ask about the military bits, to make them true. The part you might therefore want to consider is the "fantasy" part. You obviously can't organise a meet and greet with Conan to discuss how things really go down in an alternate world but you might want to see if you can digest some of Tolkein's stuff (for example) about what fantasy is all about as a genre. Compare the Cliff's Notes of that reading with your own thoughts about why you're writing a military themed story.
As for things like not having direct experience of being a leader within that scenario, that's not important. What is important is the fact that you have the experience of being led. I don't and so would find it immeasurably more difficult to talk about the relationship between soldier and commander. Your experiences are a way into reversing role situations via thought.
We are all just human beings and titles like "leader" or "soldier" are roles that we play. In order to get into the head of a leader try to remember the emotional responses you have had to leaders who were, in your opinion, "good" or "bad". Try to remember and to list the reasons that you formed those opinions. Then try to understand why they acted the way they did and treated their soldiers the way they did. Once you gain some clarity on that you will start to understand how to portray the life of a leader, at least for fictional purposes.
Were I to do the same thing I would have to start from scratch and do a hell of a lot of research. What I came up with in the end would always be from a distance and would lack the interesting features that a person who has really lived similar experiences would be able to bring to the tale.
I just read Piers Anthony's "Bio Of A Space Tyrant" series and in the second book he talks about the titular Tyrant's experiences in the Jupiter Navy. Anthony did two years service in the US Army between 1957 and 1959 and that experience shows in this volume, easily the best of the five book series.
I don't think Anthony himself particularly thought that his small sliver of military experience was of all that much help to the SF series he wrote in the mid eighties but the detail he included effortlessly gives the lie to that. The other four volumes detail the protagonist's experience as a refugee, a politician and as the Tyrant of Jupiter are not nearly as strong. They're all good fun but none of the other volumes make you understand the protagonist as much as the soldier volume. The experience was key.
I've never been a soldier, so I have no experience in that matter. But mentioning war stories and fantasy automatically Tolkien comes to my mind.
Tolkien took part at the Somme Offensive in WWI. Many people think, that he transformed the nightmare of the war into Lord of the Rings. Which leads to comparisons like Sauron plays the role of the German Kaiser. Or that the Saruman's army, based on technical power, clearing a forest to have fuel for the underground machinery, is a symbol for the German mobilisation, tactic and the way they fought.
I have no idea if these things are true, but it is easily imaginable, that he put his war experience into his books. If you haven't read LotR yet, I recommend it as inspiration.
Warriors/fighters have always a place in fantasy, no matter if it classic like Tolkien or if it has a more contemporary background. I'm sure you find a way.
On your comment above about infiltrating the Necros: The Shadowrun series is built around the idea, that in the future mega companies have taken control over Earth. They hire small teams of mercenaries (called Runners) for infiltrating (shadowrunning) other companies (industrial espionage).
I do not know, how many ex-soldiers are writing there, but as source for inspiration it's worth looking nonetheless. Maybe the series is filed under science fiction, but it is fantasy with orcs, elves, dragons, and, well, mercenaries.
Depending on the type of assistance you think you might need, there are a lot of different places you can go to find it. If you are looking for people to critique your work, I would recommend Critique Circle. It is pretty easy to get your work reviewed by other writers who will give you some honest feedback on what you've written.
If you're looking for encouragement or support, I would suggest joining a writers group or forum. There are a couple that I frequent that I have found helpful at different times. One if the Indie Writers Unite group on Facebook, and the other is the KindleBoards. The second one focuses mostly on self-published writers who are publishing for Kindle, but all authors are welcome there. The Writers' Cafe has a lot of threads that are relevant to most writers.
The Halo series was a very well received shooter game + fantasy novel series. I am certain your experience can be applied to a similar setting ie a sci fi story about a futuristic soldier.