This tutorial shows how to make holes in clothing: i.e. shorten pieces, remove sleeves, collars, etc.
We use Occlusion Maps (OM) for this. I learned about OMs five seconds ago, so I do welcome experts correcting and suggesting improvements.
Here is a regular T-shirt where I like the fit but I want it to show some belly.
Aha! But I am kidding, me actually wants underboobs.
Yes! The best underboob clothing in game
Here is how you do this:
Step 1
Export item's OM: Material Editor (ME) -> top -> OM Export Texture
This is the default one for t-shirt
Step 2
Change the texture in a graphics editor. The MS Paint does just fine-ish. Green masks, blue gives back. .Thus, more green, less clothes:
gives this
yet more green gives this
Step 3
Now import the edited texture back:
ME -> top -> OM Import Texture
Step 4
Set AlphaEx under 1. Starting at 0.99 is a great idea.
Step 5
To help you think about mapping the OM to items, here is a kit for the "top" items. If you are making OMs for other dress segments (bottom, stockings, etc.), best to make a template for each segment as the OM templates do not port from one segment to another.
Step 6
When making your own OMs you first extract the bump and the texture maps from the Material Editor to see how the program structures that particular piece. Then you paint in blue the area you want to keep and in green the area you want to mask out.
Note: these textures do not really reset or reload well at all. In case of a mistake have to pretty much start over from a project->load previous save.
Edit:
The results do depend on girl models, mods, etc., etc. If the steps above don't work for your chara, try these steps on a default chara just to ensure you got the workflow right.
Here is a regular T-shirt where I like the fit but I want it to show some belly.
Aha! But I am kidding, me actually wants underboobs.
Yes! The best underboob clothing in game 
Export item's OM: Material Editor (ME) -> top -> OM Export Texture
This is the default one for t-shirt
gives this
yet more green gives this
ME -> top -> OM Import Texture
Set AlphaEx under 1. Starting at 0.99 is a great idea.
