guideline

-model parts -weight painting -rigging -importing to toolbox -textures -materials -skeleton animations -texture animations -voice mods -ui mods -list of hair bones

physics bones are anchored by the first one (e.g. hair1) and each subsequent bone falls down like a chain (e.g. hair2, hair3).
the bones and the model's hair should be horizontal in order for this to work correctly
you must replace a character that has these bones in order for this to work -peach, pink gold peach, pauline etc.
each of these characters has a different physics strength however, for example pgp's hair doesn't move as much as peach's.

additional bones that rotate as you move (but don't have applied physics/gravity) include hairL/hairR, mustacheL/mustacheR

tour costume importing guide (no animations, model only)

In this guide, we will learn how to port an alternate driver model from Tour over their Mario Kart 8 counterpart. Note that we will not be covering animations here (yet), only the character model itself

First you'll need to import your downloaded model into Blender. Go to File > Import > FBX and select your character model. For this example I will be using Mario's Chef costume.
The character will be a bit small, so we will scale it up by pressing A (to select all), press S to scale, and type "1000" on the keyboard to make it 1000 times larger. Once your character is the correct size you can press Enter.

All we need to do here is make a few adjustments to the model so it will import correctly into Mario Kart 8.
First, if there is an ambient light in your scene collection, click on it and press X to delete it. We don't need this, only the character model.

If the character is holding any props (e.g. Chef Mario's frying pan), click on them and go into Edit mode. We will now press A to select all vertices, and go to Mesh > Merge (or M on the keyboard) and select "By Distance". This will make sure each part of our model (the head, the hands, etc) separate properly in the next step.
With everything still selected, go to Mesh > Separate (or S on the keyboard) and select "By Loose Parts". Now we can go back into Object Mode and select each part we don't need, again pressing X to delete them. Once this is done, press A to select all and press Ctrl+J to join everything back together.

Now we need to make sure the face is separated from the main model. Mario Kart 8 only allows us to have one material per object, so in order to have facial animations this needs to be a separate mesh.
Go back into Edit Mode and press A to select everything. Once again, we are going to go to Mesh > Separate (or S on the keyboard) - but this time we select "By Material". This makes sure each material gets its own mesh (in this case, the face and the body).

In order for the model to import smoothly, we need to make sure everything has the right name. This is so everything has the correct material once we import it into Switch Toolbox.
Select the body, and rename it to "Body__m_Body" in the Scene Collection. We also need to rename its mesh to match it, so let's expand our object in the Scene Collection and rename the object with a green triangle beside it. This needs to be done again for the face, but we will rename that to "Body__m_Eye".
Finally, if you go to the Material tab on the right (the red ball with a checker pattern) you will see the material highlighted in blue. Make sure for the body this is called "m_Body" and for the face this is called "m_Eye"

We have now finished everything we needed to do in Blender! Go to File > Export > FBX and save it somewhere. As with all Mario Kart 8 models, make sure to set the Scale parameter under Transform to 0.01.

Open the original character you would like to replace. In my case this would be Mario.
Click the plus next to Mario, then Models. Right click on the model whose name matches your character (usually the top one) and click Replace.
Once you've selected your model we will have a few import settings. We can press "OK" on the first menu, no changes needed. On the second menu, uncheck the "Use Material" box. We can now press "OK" on this too.

You should now see your character model in the preview window:

Notice how it is all grey? This is because we still need to replace its textures.

First we will replace its bake map, so we can see its textures in the preview.
Since Mario Kart Tour doesn't use bake maps we will need to replace this with a single color. Select the Bake texture in the Textures folder, and the image should appear in the bottom right corner of your screen. Above this, we can press Edit > Fill Color. You can just press OK here, and we should now be able to see its textures in the preview.

There's a problem though - it's all messed up! We still need to import its new textures from the Tour model.
Right click on the [CharacterName]_Alb (in my case, Mario_Alb) texture and find its matching texture from the Tour model. Do the same for the _Nrm and _Spm textures.
If the character's face looks a bit odd, select each face and replace it with the corresponding one from the Tour model. The Tour model will only have one normal map (_nrm) for the face, so when replacing the normal texture you'll need to reuse that one.
If possible it is recommended not to replace the face textures, as they will be lower resolution compared to the original ones used in Mario Kart 8.

Save this file into your mod's Driver folder, and voila! You can now play as this driver variant in Mario Kart 8!