If you're reading this guide, chances are you are a dead survivor, about to be a dead survivor, or want to give survivor a shot and increase your chances of survival. You will die. A LOT. Being prepared to die at a moment's notice is a good mindset to have as a survivor.
Everyone is out to kill you, sometimes even the marines.
The Xenomorphs hate everyone but they really hate you in particular. They can use you for additional larva or simply kill you to snuff out resistance and information should you get rescued. If you are spotted by a Xenomorph, chances are you are already dead if this happens. They'll hunt you down to the ends of the earth if they spot you.
The marines are here to rescue you, should you manage to survive that long. The sentries on the other hand, not so much. The sentries will mow down anyone they don't recognize. Be sure to get the Pilot Officers to retract their dropship sentries, lest you become mincemeat when nearing the dropship.
Should you manage to survive long enough to reach the ship, you may end up meeting the Corporate Liaison. The Corporate Liaison is a wildcard as anything can happen if you end up in their sight, from being GRUESOMELY questioned about the events on the planet and signing a waiver that says you'll never discuss said events to anyone, to being tied up to a chair and beaten with a crowbar in their inner office because the magic fax machine told them to.
Anything can kill you. Maybe the admins don't like you. Maybe those screaming, fluttering mothpeople building weapons of mass destruction don't like you. You never know what wants you dead.
You've just woken up from [insert backstory here]. You could have anything on you from a peashooter to a barreled shotgun. You'll want to adequately assess your loadout and items immediately, as well as your location as time is of the essence. Your time is VERY limited as the larva have a short period of time before they evolve and begin the hunt for YOU. Grab any supplies around you that you can and head out or bunker down at the location you woke up at.
Building a small defensive base is a good way to increase your odds of survival. Certain locations are high traffic areas your average Xeno will check when hunting for survivors. These areas can be anything from the dropship landing zones, the map's armories, the Telecommunications area, or cargo and general areas with bountiful supplies. Should you set up in a lower traffic area, the chances of a Xeno finding you before rescue are lower, but they WILL find you, and when they do, they will shriek to their Hivemind, bringing every blood-frenzied Xeno over to assault you. The more difficult it is for a Xeno to travel from the Hive to your base, the better.
You've found where you want to set up the base and have it properly set up. This includes barricades, barb wiring said barricades, razorwire, welded doors, and welding vents can stall or deter a Xeno. Eventually, you will get spotted by a Xeno, most likely a Runner or Hunter. This will be your best chance at striking the Xenos. Chances are, the area you've selected has no weeds nearby, and if it did, hopefully you managed to clear them. If the Xenos aren't idiotic, they'll call a Drone to weed the area near your base, as well as a caste with medium acid to melt your barricades. Should the area be weeded, the Xenos will be able to relentlessly assault you. Should they have no nearby weeds, hurting the Xenos will cause most of them to retreat and heal, stalling the assault on your base.
Perhaps the Hive REALLY hates you. You'll know they're pissed when tier 3 castes are assaulting your base. Unfortunately, this is a losing battle as there's not much you can do to stop the tier 3 castes from busting your barricades down and annihilating you, but have hope! The appearance of tier 3 castes means time has passed considerably and, should the marines not be held up on the ship for whatever reason, rescue will soon arrive.
Sometimes you might find another survivor or three out there. Teaming up with the other survivors is a good way to increase your survivability but at the same time, can result in a liability as you'll have to cover their ass as well as your own. Always try to coordinate with the other survivors as best you can. Teamwork is key.