It depends on what you're trying to do. What are you wanting to make happen?

When the official forums come back up then I can point you to a do-it-yourself mod for some things, made by Smashly. Other than that I would say the only easy way is editing the files you want to change if it is something basic like changing character skins or messing with a few variables.

Added cowcar to release!

You'd have to build the folder structure since apparently the original person didn't do it. Generally when we build mods we build the structure for you :) The basic idea is that the engine works on a kind of compacting system so you won't normally have those folders there. So make the folder "scripts" in your game directory, then make "entities" inside "scripts" then "items" inside that, then "xml" in that then "weapons" in that. Then place the files all the way down in weapons.

I assume that's what you'll be after :)

Added Armored Vehicles Mod for Black/Red Truck, currently. Coming soon is one that replaces the black muscle car with a cowprinted Mad Max style vehicle. See post for download link, or check out the fuller post here:
http://steamcommunity.com/groups/stateofdecaymodding/discussions/1/666827316004239423/

Note: I don't remember to come here often so this may be out of date at any time - see the main thread about it here on the UL forums:
http://forums.undeadlabs.com/threads/fortitude-mod-vehicles-armor-and-apocalypse.37797/

Screenshots of the Hatchback:
http://steamcommunity.com/sharedfiles/filedetails/?id=201911076

http://steamcommunity.com/sharedfiles/filedetails/?id=201911237

Screenshots of the cowcar:
http://steamcommunity.com/sharedfiles/filedetails/?id=199754150

http://steamcommunity.com/sharedfiles/filedetails/?id=199992981

http://steamcommunity.com/sharedfiles/filedetails/?id=199993216

Comes with a coupe variant that replaces the green muscle car with a version that has only 2 seats, no rollbar, and no back seat.

Screenshot of Truck:
http://steamcommunity.com/sharedfiles/filedetails/?id=198948761

No worries, it's kind of a weird sertup when you look at it abstractly compared to a wide-open setup that one is used to. It's good for the game running, not as easy to mod though :)

You just save over the old DDS, you shouldn't ever have to dump things in the PAK since the game will pick it up as long as it is in the right directory being pointed to.

I don't know anything about save game modding, nor have I tried :)

Snyder should be released this weekend but I'm going to point people to my post on the forums because I think it's better to keep mods there since it brings traffic to Undead Labs and provides more of a community with mods :)

http://forums.undeadlabs.com/showthread.php?36077-Fortitude-a-State-of-Decay-base-stabilization-Mod-by-Phacops-Eocryphops/

Note that I've already developed one armored vehicle and that will be released at the same time as a beta. It will be invulnerable to all damage, including being turned on its back, so only use it if you wish for that, otherwise wait until I release the fixed version, please.

I edited some files and achieved most of them disappearing but I'm not pursuing it further because Adeno has already completed it, they say, and should release it soon -

http://forums.undeadlabs.com/showthread.php?33990-George-Romero-Mod-(Experiment)/