You'll need Blender[www.blender.org] and Blender Source Tools. I currently have the Steam version of Blender, so I expect updates to break the plug-ins sometimes, however you can always install older versions of Blender through their website.[www.blender.org]

https://steamhost.cn/app/365670/Blender/

You'll also need fragMOTION[www.fragmosoft.com] for B3D, SMDs, the app also has support for other games like Doom 3 from what I've heard. If you're exporting an FPSCreator model, that being the D-Class, clerk, or zombies, you'll need an older version of fragMOTION.[gamebanana.com]

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3491970800

If you do not use an older version to export the zombie skeletons, this will potentially happen.

Find a character's B3D file at then import it into fragMOTION.
Note that the file icon preview may be different from the screenshots.
You'll then have to export it as an SMD file to put into Blender later, this'll correct the skeleton.
I put it into a new folder located in my Downloads folder for this tutorial.
Exporting the animations is optional, but if you want to adjust them and their poses, you can export them as a separate SMD file, but select "Animation" instead of Mesh.
The SMD Export window will pop-up after selecting the folder location.

Open your Blender software, and if you have Blender Source Tools installed, you can import the SMD's mesh and skeleton for use.
I've decided to use this creepy teeth monster I made a bit ago for an example.
If your new mesh was parented to a different skeleton previously, you can head to the right and click the green icon for Vertex Groups, then delete unlocked groups before the next step.
Select the new mesh first, then select the skeleton whilst holding "Shift" on the keyboard.
Parent the mesh to the bone using Automatic Weights.
You can adjust the Weights using the Weight Paint later if stretching occurs.
Currently his arms and legs are off-center, so I went into Pose Mode and moved the limbs.
Apply the pose by selecting the bones with "A" and then pressing "I" on the keyboard.
I then went into "Modifiers" on the right and clicked "Apply" in the drop down.
This unparents the mesh from the skeleton, but it applies the pose to the limbs.
I then used "Clear Transform" to set the pose back to default.
Then I parent the mesh to the bones again, like previously.
Delete the old SCP-106 mesh, but keep the skeleton.
On the right, go to "Scene Export" then select a folder in which you want to export the SMD.
Don't forget to switch it from "DMX" to "SMD" and have the new mesh selected.

Import the SMD file, the same way to import a B3D file into fragMOTION.
Then "Merge Animation" with the older B3D file from the game.
Do not use the "Import" button here as it will replace the entire scene.
Again, here's the folder location for the game's B3D models.
Then export as B3D file into a Workshop folder.
Do not put this anywhere else before the "YourAddons" folder, as the Workshop folder will just outright delete it on the game launching.
Before you export your B3D file, don't forget your texture directory in the material as well, unless you want it as a blank base material with one color.

Create a new text file and set it as "test.gs" or any name, it must be GS file so you can compile it later. Here's my example here which'll replace the model and texture image.
When they're replaced, the game moves the older files into the "BackUps" folder for use later.
Drop the GS file into the "ScriptsCompiler" folder here.
Then open the GS file in Notepad and type below.
I put the JPG here, as I put my new texture JPG into the same folder as the model itself.
You can also refer to the GS/GSC file guide below in the guide sources section.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2668197570
Open the "Compiler.exe" file and type the command "Compile test.gs" then hit "Enter."
This'll compile the GS file into a GSC, once this is done type "Exit" or close window.
Drop your GSC file into "YourAddons" folder for testing in-game.

Now that the GSC file and everything is in the "YourAddons" folder, start the game.
I used the console command "Spawn 106" below as the monster starts chasing me.
Once you confirm everything works as intended, drop your mod into "WorkshopContent" folder.
There should be some example folders in there as well, such as "DClown" or "Test" to help.
Then use "SteamWorkshopUploader.exe" to upload/update a Workshop mod.
Don't forget a unique image thumbnail to stick out better! Use a 512 x 512-sized JPG.
You can also add tags for models, pictures, and textures.
Reminder that if you use other people's works, please do credit them in your description.
This includes custom animations, models, textures.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2668197570
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3030462899
https://youtu.be/UfWWKCXoJ38
https://youtu.be/eYug3WVpgcQ