Quick introduction:

I thought the leather jacket + shorts outfit (for adult females) that came with WA was unbelievably ridiculous. The leather jacket looked FINE on its own, and then EA had to go and pair it with the oddest-looking shorts I'd ever seen. I wanted to do something about it, but I had no idea where to start - and none of the tutorials I found were of any immediate use to a beginner like me.

Can someone explain how I would go about changing the outfit to top-only? Any help would be greatly appreciated!

Not the easiest thing to do for a noob im afraid...
You first need to extract all necesarry files, this means you have to open the original packages with Postal(or S3pe?) and extract:
The original WA EP textures, the base/specular/mask and normalmap file
The Meshes and its morphs, a total of 15 GEOM meshes/files

You need to open the LOD1 base mesh(and its 3 morphs, or 4 when pregnant is there also.
Then import a basegame LOD1top(and its morphs) and cut the fullbodymeshes in half and fix the waist when needed.
You can do this by using the basegametopmeshes as reference(use Wes H his merge tools)
Delete the basegame meshes when done and export the custom top meshes(base/fat/fit/thin and maybe pregnant)

You do this again with all LOD2 GEOM/meshes(with the help of a basegame lod2 top and its morphs)
Then again for the lod3....

When you have all 15 meshes done you can make new Bgeo and Bblend files with those.
Use BMM(bodymorphmaker) for this, its not that hard.
But before that i personally recommend you clone a basegame top with CTU and load all necesarry textures in it and also load the 3 LOD GEOMs in it, the ones you just created, the base LOD1/2 and 3(not the morphs)

Save that package and open it with Postal and write down the 4 instance numbers from the Bblend files.
With those instance numbers you create the new Bblend files and Bgeo's
You can do this with Postal and Bodymorphmaker both open at the same time.

Import your new Bgeo files into the package(check its instance ID, should be the same as the Bblend files)
Then go to the Bblend files(in the package) go to the Hex Tab and import the new Bblend files)
With Postal still open there is one thing you need to fix and those are the TGI references from your custom LOD1/2 and 3 base mesh.
You need to open those and give them the same TGI references as the top mesh you just cloned with CTU Save it...
It might be difficult the first time you try this but once you have done it a few times its not that hard.
I normally have all 4 programs open(Milkshape/Postal/BBM and CTU) and switch between them so i can fix the TGI's before cloning and saving the top in CTU.
Plz correct things if i forgot anything.
Some one should do a tut for it :D

Edit,
Oh i forgot the normal map
I think in this case its easier to link the mesh to a blanc normalmap, a basegame normalmap texture from a shoemesh will do fine.
You need to write down a basegame blanc normalmap number(group/type and instance and change the mesh its TGI reference(group comments) in it.
Hope this makes sence :s