Create a new 3D Unity project.

Download the UniVRM unitypackage: https://github.com/vrm-c/UniVRM/releases
Make sure the package is compatible with your version of Unity.
This will allow us to import our VRoid model into Unity so that we can start editing the model.


In Unity at the top menu go to Assets -> Import Package -> Custom Package.

Select the Unity package file you just downloaded.

Click Import at the bottom of the Import window.

You should now have folders called UniGLTF, VRM, and VRMShaders.

Now that we have UniVRM imported we can start importing a VRoid model to Unity. I will be using a VRoid model from the stable build of VRoid Studio. Again, this guide should work for VRoid models from both the unstable and stable versions of VRoid Studio.

Create a new folder in the Project window under the Assets folder and call it Prefabs.
(Right click -> Create -> Folder)

Create another new folder in the Prefabs folder called after your VRoid model's name. We do this because this part is going to create a lot of assets for your VRoid model that we want to keep organized since you might want to import more VRoid models to Unity in the future.

At the top menu go to VRM0 -> Import from VRM 0.x.

Find and open your VRM model.

Save your model's prefab in the Prefabs folder.

Find your model's prefab in the Prefabs folder in the Project window and drag it into the Scene window. It should have an icon of your model T-posing.


(Optional)
Your model might look a little yellow. Just click on the Directional Light in the Hierarchy window and change the color to white and up the intensity a little bit.

Drag your FBX file and material file (PNG or JPG or whatever you use for the texture) into the Assets folder to import it into the project.

Drag your accessory model from the Assets folder into the Scene window.

Right click -> Create -> Material.
Name the material something.

Click on the newly created material.

Drag your image texture on to the small box where it says Albedo.
Drag your normal texture (If you have one) on to the small box where it says Normal Map.


(Optional)
At the top of the Inspector window click on Shader then select VRM -> MToon.
We do this give the material the same shader as your VRoid model.

Drag the material on to the model in the Scene window.
Repeat this for every part of the mesh.

Drag your OBJ file, MTL file, and material file (PNG or JPG or whatever you use for the texture) into the Assets folder to import it into the project.

Click on the OBJ file in the Project window and go to Materials in the Inspector window.

Set Location to "Use External Materials" and set Naming to "Model Name + Model’s Material".


Click Apply to automatically add its materials to the accessory.

This should have created a new folder called Materials.

Go to the materials folder and select your material.

In the Inspector window make sure that the Albedo color is white.

(Optional)
At the top of the Inspector window click on Shader then select VRM -> MToon.
We do this give the material the same shader as your VRoid model.

Drag the accessory model from the Assets folder into the Scene window.

At the top of the Hierarchy window in the search bar type the name of your desired body part/bone. For example, if I type "head" it will show up with "J_Bip_C_Head".


Select it and then clear the search bar.

Drag the name of your accessory on top of the name of the body part/bone you want it to be attached to in the Hierarchy window.

Click on the name of your accessory in the Hierarchy window and adjust the Transform in the Inspector window. You can position, rotate, and scale your accesory there.

Select your model's name in the Hierarchy window.

At the top menu go to VRM0 -> Export to VRM 0.x.

Fill out Title, Version, and Author.


Click Export at the bottom.

Save the VRM model somewhere else where you can find it.

Enjoy your new accessories.


Please consider upvoting this guide if you found it useful as it helps with visibility for other users.
Feel free to ask me if you need help.