The "how to" discussion regarding AFMBE came up, again. This time, I typed up a different response, but linked to the first one here on the blog.
All of this is YWWV. There is no tl;dr and I may wander all over the place. Another treatise on this is here.
If you can, read One of the Living and steal everything you can from it. If you steal nothing, read it and let it sink in. There are gems in there, even if you (and I) don't see them at first.
If you need a starting point, download the free demo scenario and use/modify it or purchase the Zombie Master screen and use/modify the scenario in there. It's nice to have someplace to start.
Sketch out an outline of where you want the story to go. For me, this means locations. Set milestones (signal flares, goal posts, whatever) of where they need to go.
Rise starts in a bank
Tease with a safe place
Escape to make-shift hideout
Escape town, hideout in the country
Join up with military base
Head for the coast
Don't just focus on the zombies. The real enemy is other people. Have the characters meet plausible NPCs who are either interested in joining the characters, avoiding the characters, or taking things from the characters. Make sure you know why for each of them. Not all bad guys are as big of murder hobos as the characters, but some of them are. Start slowly with the NPCs and build up to more, then wipe out a bunch of them. Rinse, repeat.
Look up spooky and abandoned places on the Internet, use them.
Google Maps is your friend. If you can pull up views of Google Maps during game play, half your work is done. No, the characters may not have access to the Internet, but your players do. The characters would likely know the town they are in, so let the players use Google Maps to know it, too. Let them do the heavy lifting of where they want to go, you just need to have encounters ready.
Know what caused the rise, how to cure it (if it can be cured), how it spreads, and how quickly it will spread. Does it take death and then they come back? Does it take infection and then in "X" hours they turn? Is it a combination? Is it supernatural in origin and only certain people turn? Are certain people immune? If so, what causes the immunity? (The "greeen flu" in Left 4 Dead is not contagious to everyone, including the characters. They only die from trauma.)
Know everything about the zombies you plan to use. How long does it take them to decay? will they decay? Will they develop other abilities over time? If so, start thinking about what those are now, not later. Drop hints at it now. So, if they will develop claws or talons over time, make sure one of the characters gets scratched up real bad during the rise. It may not infect them, but they know those sharp nails exist.
How is the government responding? How do they lose control? The same general idea for the rest of the world. It's something for the back of your mind.
What zombie movies, TV shows, or other media do your players like? Can you work in a single scene in your game that is similar to one single scene from a movie? Similar, not exactly the same. fit it to your world.
Steal from everything you find. Left 4 Dead has specific locations in each adventure. You move from point a to point b. You start at point b and move to point c. It's an easy set up and something that many Zombie Masters don't do. It's structure. Even if they stay in the abondoned train car for a few weeks, eventually they will need to move on.
When the world ends, noises carry. Generator hums can be heard a mile away. Gun shots carry for a long ways. Car engines echo. Bad guys, desperate families, and the dead can follow those sounds (or ground vibrations).
Hurt them. Physically. Mentally. Emotionally.
Emotionally: If they lose track of their family, make them deal with it. Maybe they cross off their family and it leads to their Batman/Punisher/Murder Hobo ways. Maybe they insist on breaking from the group to find their family. That's fine, let them, and go through it as quickly as is possible. A single roll or two to find them (Per or Int + Notice or Area Knowledge type of skill and if necessary a combat or drive test to get there). Let them fight through the danger on-site and discover they are either alive or dead. Now get them back to the group. Let the other players watch the situation and be just as immersed as the other player. It's gold for buy-in into the setting.
Mentally: This shit breaks you down. where do we go? What will be there? What will be in the way? When do I get to sleep and who do I think will make a good watch person to make sure I don't die? Make them roll Fear Tests. If they have silly things in their background such as liking to take risks, make them roll Willpower to resist doing something really, really stupid - I mean awesome! Where can they find food? Who else will be there looking for food?
Write down their Qualities and Drawbacks. Make yourself make them role play through the situations where their Q/Ds are appropriate.
Physically: Unisystem can be quite deadly. Let all of them take Hard to Kill. In fact, if they have any Quality points leftover and they haven't maxed out on HtK already, make them do it. Limit all of the other crazy, survival bonuses to 1 or 2 characters at most. No, you can't all have Fast Reaction Time. No, you can't all have Situational Awareness.
Do not let them have machine guns and the like. No one will leave them sitting around for random characters to pick up and they'll elevate the violance in the game to a level that is not sustainable and filled with survival horror. A 9mm pistol, a baseball bat, a hockey stick, a shotgun? Sure. Hand them an ammo sheet and remind them to mark off a bullet every single time they shoot that gun. Every. Single. Time. They will eventually tell you they are marking it off before you remind them. Until they do, "Lose a bullet from your ammo sheet." You can find those ammo sheets here.
Describe the damage they recieve. It's not just you got shot. It's you take 12 points of damage. How much does your bullet proof vest abosorb? Okay, so that leave 4 points through, which doubles to 8 once it hits flesh. How much are you down to? Then, you can tell them where the bullet entered and *if* if left the body. It's also the baseball bat hits you on the left arm as you try to dodge out of the way. You take 20 points of damage. Make a Difficult Strength test to avoid dropping your weapon. You failed, you hear a crack and feel unbearable pain as the bat breaks your arm and you drop your gun.
Keep in mind that combat kills. By keeping the weapons on the lower end of the damage scale, you can keep it somewhat realistic and the characters might survive. Remember, nobody wants to die. Not the characters, not the NPCs they are fighting. Some people might actually give up or run away. The most intense AFMBE combat I ran had 2 PCs on 2 NPCs. The NPCs had a shotgun and a baseball bat. The PCs had a .38 and a machete. The PCs thought they had the situation under control with the baseball bat NPC, until the shotgun NPC spun out from behind a tree and put the PC with the machete on the ground with one shot. The PC with the .38 got in a lucky shot on the shotgunner, but then the baseball bat NPC attacked the PC with the .38 with punches and kicks (he had been disarmed). There were several botched rolls and eventually the NPC climbed a fence into a neighborhood and got away.
And running a good combat scene is hard. It's just as hard as plausible NPCs. However, mediocre combat is easy, if not as emotionally intense.
You need to decide if your game will involve PC death. If it does, make sure you players understand this. If it does and they are newer to Unisystem and AFMBE, make sure they understand how much damage they can do with a weapon you expect them to see in the game. When I Zombie Master at conventions, one of the things I do before we jump into the game is have everyone roll damage with any weapons on their character sheet. Then I have them compare it to their Life Points and ask how many are below 50% of their starting LPS and how many are nearly dead. This puts it in perspective.
Don't forget about the chaos swirling around them. Car crashes. Fires burning out of control with no fire department to douse them. Crazy drivers trying to get out of town. Police and military trying to contain everything - people, zombies, disasters - and being overwhelmed.
There is one final character, the world. In Westerns, the land is often a character. I tend to do this in games of AFMBE, as well. The land and the animals will take back the cities. They will run amock in the fields. This will bring back predators such as the wolf. It will expand the coyotes' already large hunting areas. Bring in coydogs, coywolves, and wild dogs or mixed packs of all three.
I include weather with the terrain. Torrential rainfall could lead to flooding. It certainly drowns out sounds of movement. Buildings decay and collapse. Radiation zones are dangerous in ways most people cannot fathom. Drought kills crops and that affects meat food sources, too. No water or sunlight, no vegetation. No vegetation, no bunnies or deer. No bunnies or deer, no predators. None of the above? No people.