6. Weight Painting / Skinning

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Getting rid of clipping is by far the hardest step.
You'll be working with Vertex Groups in Weight Paint Mode.

Vertex Groups determine the influence each bone has over part of the mesh and our goal here is to very precisely align influences of clothes to those of underlying body so they would move in unison.

Side note: In XIV weights are used not only for animation, but for racial scaling as well, meaning there are predetermined matrices for every given vertex group that transform it a certain way and that's how midlander female magically becomes femroe body without requiring it's own model.

Pic. List of Object's Vertex Groups

Transferring Weights

It's so precise in fact that main bulk of the work is done by just letting software project weights from body to clothes. To do this properly in Blender select your destination object and go to Weight Paint Mode. Then Ctrl+Click on the source object to add it to selection and press Weights > Transfer Weights.

Pic. Selection for Transfer

Then select
Vertex Mapping: Nearest Face Interpolated
Source Layers Selection: By Name

And done, now your destination object should have all the weights projected from source

Fixing potential issues

For the most part transferring works surprisingly well, but two common problem areas are crotch and breasts. For clothes that just loosely hang between breasts you wanna blur the center line, so influences from both breasts would overlap a little, smoothing the transition.
Use either Blur Brush or Smudge with low strength and don't forget to enable Symmetry. Breast bones are called j_mune_r and j_mune_l.

You wanna do it because what breast slider does in-game is it scales the breast bones up and that leads to pinching in that area.
And even when the slider is at 50% it's still gonna pinch from racial scaling.
By the way, to simulate breast slider in blender scale the breast bones to
X:1.08 Y:1.2 Z:1.184 for 100
X:0.922 Y:0.804 Z:0.82 for 0

And with crotch area being so tight you may want to test how it handles the legs spreading apart. For that you can select the armature, go to Pose Mode, spread the legs and then return to Weight Painting Mode to fix any possible issues.

General Tips

1. What isn't there can't clip. Delete parts of the body that are heavily obscured by the clothing. (But make backups first)
2. You don't always have to it. With weights being such a pain, the less you have to mess with them the better.
   For example if you're upscaling an item and it doesn't have any visible skin elements (pants etc.), it's best to just leave the vanilla weights alone.
3. Clothing bones usually control stuff like shoulder pads and they aren't skin tight. It's better to cut these parts out of your objects before transferring to preserve the stock weights. You can later join them back together.
4. Custom Gradient in blender settings.
   You change how weights are displayed and a useful trick is setting a custom gradient in Edit > Preferences > Editing > Weight Paint
   
   With this scheme:
   Pos 0.000 - It's own discernable color when the value is 0
   Pos 0.001 - Start of the gradient, e.g. Black
   Pos 0.999 - End of the gradient, e.g. White
   Pos 1.000 - Again a separte color when the influence is at 100%, e.g. Red
   Another official way of doing it is by setting Zero Weights to Active, but I like gradient method more
5. Weight Contours, turn them on to better see the falloff patterns.