Ahh man. wrong answer LOL
Well if its sleeveless then extrude from that point, its the seam where the sleeves would be sewn if it were a real garment.
If it were a jacket cut the front faces out and just add some thickness so it doesnt look like paper.
The principles of modelling the garment are the same for either of these objects.
I take it you have a character already modelled?? Then use that as a template, dupe the character then extract the body and arms out of the mesh, you have a garment pretty much done , a few tweaks to the mesh to make it look like a garment and you are sorted.
Really, it could not be any easier, I think you are looking to be too technical for your own good.
J
Yes, i already created the character (and it's smoothed too), i tried that method with copying few days ago and decided that i didn't like the way it looked.