Creating Custom Backgrounds

Custom backgrounds can definitely help your levels look great. This tutorial will explain the basics of importing your own bg models.

Prerequisite

You will need:
Switch Toolbox
Blender
Global Mat File

Exporting Models

First, you'll want to export the background model you want to use as a base.
In Switch Toolbox, open the file of the background you want to base your new background off of (A list of each background and its description can be found here.).
Double-click the BFRES, open the Models folder, right-click the model and export as .dae.

Import the bg model into Blender and scale the model down by 0.01. [If the model shows Z-Fighting, increase the Clip Start by x10.]
Then add your own models into the bg but be mindful of any model-related limitations the game may have. [Make sure your models have vertex coloring or else they won't display properly.]

Parenting Your Model

1. Highlight on your model(s) in object mode and then shift hold click the armature.
2. Click Ctrl + P and click the button With Empty Groups.
3. For every model, highlight the model in edit mode and assign the model to the root bone in the vertex group tab. [Root bones are mostly named the bg you are trying to add models too.]

Importing into Switch Toolbox

Highlight only your model(s) and export as .fbx while also ticking the Limit to Selected Objects checkbox. [Make sure the scale is 1 when exporting.]
Back on Switch Toolbox, right click on the Textures folder, and click the import button.
Import all your the textures and make sure the format is set to T_BC3_UNorm.

Go to the Objects folder, right-click and select Import Static Object to import your .fbx.

On the Import Settings window, click Ok on the first window and on the second window on the model tab:

1. Uncheck Create Placeholder Textures.
2. On the Use Material box, click on the box and select the global material.

Replace the textures in the materials with your own textures and be sure that your models have the right materials assigned.
Texture order should be:

1. lm_01m
2. lm_02m
3. [AddTextureHere]

After everything is finished, you can save your file and test ingame.