Arrimus 3D has a very nice shirt modeling tutorial which makes some easy work of the collar area and happens to be exactly the same technique I would use personally. You will see that this tutorial is for 3dsmax, but do not be discouraged to watch it. The collar area is just a matter of some very simple and rudimentary polygon extrusion techniques which you can translate over to Blender workflow without any confusion. Watch how he does it at the 10:35 point of the video, and I'm sure you can get the same or similar results in Blender.

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