How to convert a single-player GTA 5 car mod to a FiveM add-on

1. Open the mod's dlc.rpf in an archive tool and extract everything.
2. Create a resource folder with stream and data subfolders.
3. Move the model and texture files (.yft, .ytd) to stream.
4. Move the .meta files to data.
5. Write fxmanifest.lua: each meta under files and in a data_file line.
6. If it is a replace mod, rename the files and write a vehicles.meta entry for the new name.
7. Add ensure to server.cfg, restart, spawn the car and read the server console.

An add-on car for single player comes as a dlc.rpf archive that the player adds to the game's dlcpacks folder. Inside it are the same files a FiveM resource needs, wrapped in the single-player DLC structure:

dlc.rpf
  content.xml                     list of every file and its type
  setup2.xml                      name and load order of the pack
  data/                           (or common/data/)
    handling.meta
    vehicles.meta
    carcols.meta
    carvariations.meta
  x64/vehicles.rpf                (or x64/levels/gta5/vehicles/...rpf)
    mycar.yft
    mycar_hi.yft
    mycar.ytd

Folder names vary between authors. The file that tells you what is what is content.xml: it lists every data file with a fileType, and those type names are the same ones FiveM uses in data_file lines.

<Item>
  <filename>dlc_mycar:/data/vehicles.meta</filename>
  <fileType>VEHICLE_METADATA_FILE</fileType>
</Item>
<Item>
  <filename>dlc_mycar:/data/carvariations.meta</filename>
  <fileType>VEHICLE_VARIATION_FILE</fileType>
</Item>
<Item>
  <filename>dlc_mycar:/%PLATFORM%/vehicles.rpf</filename>
  <fileType>RPF_FILE</fileType>
</Item>

In the dlc.rpf	In the FiveM resource
.yft, .ytd inside vehicles.rpf	stream/
Tuning part models, in their own archive when the car has them	stream/
.meta files	data/, each with a data_file line of the type given in content.xml
content.xml, setup2.xml	Not used. fxmanifest.lua does their job.
Text labels (.gxt2)	Not used. Add the name with a script line, see below.

FiveM does not load .rpf archives from a resource, so the files have to come out. Any archive tool for GTA V works. In CodeWalker, start the RPF Explorer, use File > Open Folder... on the folder that holds the dlc.rpf, open the archive and the archives inside it, select the files and extract them with Extract Raw... or Extract All.... OpenIV does the same job.

Archives are nested: dlc.rpf contains vehicles.rpf, which contains the models. Keep going until you reach loose .yft, .ytd and .meta files. The two tools are compared in CodeWalker vs Sollumz.

Arrange the extracted files like any vehicle resource:

resources/
  mycar/
    fxmanifest.lua
    data/
      handling.meta
      vehicles.meta
      carcols.meta
      carvariations.meta
    stream/
      mycar.yft
      mycar_hi.yft
      mycar.ytd

Then write the manifest. Every meta is listed twice: once under files so the server sends it to players, and once in a data_file line so the game loads it as the right type.

fx_version 'cerulean'
game 'gta5'

files {
  'data/handling.meta',
  'data/vehicles.meta',
  'data/carcols.meta',
  'data/carvariations.meta',
}

data_file 'HANDLING_FILE'          'data/handling.meta'
data_file 'VEHICLE_METADATA_FILE'  'data/vehicles.meta'
data_file 'CARCOLS_FILE'           'data/carcols.meta'
data_file 'VEHICLE_VARIATION_FILE' 'data/carvariations.meta'

fileType in content.xml	File
HANDLING_FILE	handling.meta
VEHICLE_METADATA_FILE	vehicles.meta
CARCOLS_FILE	carcols.meta
VEHICLE_VARIATION_FILE	carvariations.meta
VEHICLE_LAYOUTS_FILE	vehiclelayouts.meta

The full list of types and the order they load in is in fxmanifest.lua and data_file for vehicles. Starting and spawning the resource is in how to add an add-on vehicle to a FiveM server.

A replace mod has no metas and no new name. Its files are named after a stock car, for example sultan.yft, and the game draws them instead of the original. Streamed from a FiveM resource they work the same way: every Sultan on the server changes. To keep the stock car and add yours next to it, give the model its own name.

1. Rename the files. The file names are the model name.

   sultan.yft      ->  mycar.yft
   sultan_hi.yft   ->  mycar_hi.yft
   sultan.ytd      ->  mycar.ytd

   The textures inside the .ytd keep their names. The dictionary is found by its file name, not by what is inside.
2. Write a vehicles.meta entry. Copy the entry of the replaced car from the game's own vehicles.meta and change modelName and txdName to the new name, plus gameName for the display name. Leave handlingId, layout and audioNameHash as they are and the car drives, seats and sounds like the original. Every field is explained in vehicles.meta explained.
3. Add a carvariations.meta entry with the new modelName if you want your own colour combinations. See carvariations and carcols.
4. Give it its own handling only if you want to tune it. Then copy the handling entry too, with a new handlingName, and point handlingId at it.

The display name comes from a text label. In FiveM one client script line adds it:

-- client script: readable name for <gameName>MYCAR</gameName>
AddTextEntry('MYCAR', 'My Car GT')

Symptom	Cause	Where to look
Console warns about oversized assets	Single-player mods often ship 4096 textures and one heavy mesh	Optimising a FiveM vehicle
Tuning parts from another car appear	Two carcols.meta files use the same kit ID	carvariations and carcols
Car does not spawn	A meta is missing from files or data_file, or a name does not match	Vehicle troubleshooting
Some textures are missing	The mod relied on a shared texture dictionary that was in another archive	Add that .ytd to stream and keep the parent link in txdRelationships.

One more check before you put somebody else's mod on a public server: read the author's terms. Some single-player mods do not allow redistribution or use on servers.

A car from another game or a plain 3D file has no skeleton, no GTA shaders and no collision, so there is nothing to extract and rearrange. It has to be rebuilt as a GTA vehicle in a 3D tool first. That workflow is in how to make a FiveM vehicle in Blender with Sollumz.