How do you make a bodyslide file for an in-game item?

[LucidLady]

Apologies if this question doesn't belong in here.

 

I'm trying to figure out how to make a bodyslide file for a clothing item that's in the game?
For example - the bowler hat.

I already have NifSkope and BAE. I can single out the Bowler Hat nif and place it in Data/Meshes wherever it goes.

But at the moment I can only kinda replace nifs with other nifs. I can make Hancock's Tricorn Hat look like the Bowler Hat, by replacing the nif. But i'd like for them to each have their own separate file.

 

I'm basically just confused, here. I probably didn't even explain what i'm doing very well.

If anyone has some simple step-by-step guide, i'd love to hear it!

[JimmyRJump]

I'm not a modder, but I thought that to add an item to the game you needed to make a plugin that contains the item. For that you need the Creation Kit.