This isn’t a strategy guide so much as a few points that players new to Long War should keep in mind. Some of the mechanics can feel like gotchas if you don’t have any idea how they work.
If you have not played War of the Chosen, definitely play a campaign or two of that first. It introduces a lot of new (and very fun) mechanics. Trying to learn the War of the Chosen stuff alongside the Long War stuff is a recipe for feeling overwhelmed and confused.
I have some links to articles and videos at the bottom. I extremely recommend looking over some of them before you start your first serious campaign (If you want to start a throw-away campaign first just to get the hang of it and have fun, you absolutely should. It’s what I did! Just be aware that if it seems totally unfair and punishing, taking a look at these resources will get you on the right track)
Long War is hard. It is recommended that you play 2 levels below your usual level. There is no shame in playing on Rookie. If it is your first time out, Rookie will still be plenty challenging. You can always increase the difficulty mid campaign or start a new one with whatever strategy knowledge you gleaned from your Rookie campaign.
I had an Ironman/Legend campaign in War of the Chosen under my belt, and I wrote almost this whole doc just attempting to win a Commander difficulty LWotC campaign. It took me many tries.
In Long War most missions require you to infiltrate. This means you equip a team and send them out and they spend a number of days infiltrating. You can usually launch the mission before they reach 100%, but you will face many more enemies.
It is very handy to see how this works: https://www.ufopaedia.org/index.php/Infiltration_(LWOTC)
Generally speaking, you want to get your infiltrations to 100%. Often this means not taking missions.
You cannot do every mission. You should not try!
You have to be choosy about which missions you go on. Consider the expected number of enemies, and the value of the reward (or penalty for not doing it).
Unless it is absolutely critical, you should not take missions that don’t give you enough time to fully infiltrate a squad of at least 5. You can survive a dark event happening, but a squad wipe is hard to come back from.