amen to what forseti said
coincidence, i just finished a victorian blouse that required a high colar
i thought making it direct on the neck with the shirt template, but as i prefer the real 3D look to complete it, i decided to create a texture in Photoshop, and then apply it to a hollowed cyllinder, with some aspects like twist, cut beginning and end, etc, to make it be exactly what I needed
should work it out that way!
BUT
if you do not want a prim, and want the colar to still have a feel of 3D, best to do it, use a totally different layer (but in the same PSD file of your shirt) draw your colar, (make sure rest of layer is tranparent) and then go in layer, style, drop shadow
then you can play in the drop shadow options to obtain the exact shadow look you like
it will only put a shadow under your colar, since it was made on a different layer, and make sure that colar layer is on top of the rest of your shirt
then the shadow will appear on top of the shirt, and will make it look some 3D
thats it

have fun, hope it works for you!
elka