|I used this technique for my McCall's 4219 Tunic With Cowl (there's a review posted on this site). The picture's posted above. This particular cowl is very large -- it makes into a hood. However, the same attachment trick can be used for any similar collar -- even adding a turtleneck to a crew-neck shirt.
The collar connects with tiny elastic loops to small buttons sewn inside the neck edging.
-- 5 to 8 small, flat buttons
--1/8 inch elastic (I prefer flat elastic, but oval could work)
First, finish the neckline of the garment as usual. Then sew 5 or so very small buttons to the underside of the neck edge. When using 5, I tend to place them on each shoulder seam, one at the center back, and the remaining two spaced evenly on each side between the shoulder seams and the center back.
For a big cowl like the one in the picture, I don't put any buttons in the front, since the cowl will drape nicely and there's no need to attach it there.
Cut an elastic loop for each button. To determine the size, fit the loops to the buttons on the garment -- you want a snug fit, so that the collar won't unbutton, but not one so tight that it's impossible to get the collar on and off easily. The loops should not show below the lower edge of the collar.
Inside your collar, sew the elastic loops to the underside of the lower edge, along the hem. Attach these loops so that they are caught only at the upper (cut) edge of the elastic.
You want the loops themselves to "flap" so that the neckline of your garment slips under the lower edge of the cowl, with the loop reaching under the garment edge to the button. This gives you a smooth seam-line line where the cowl joins the garment.
If using a smaller collar, you'll probably want to add an additional 3 buttons to the front neckline of the garment, and corresponding elastic loops to your collar.
I really enjoy doubling my options like this -- I get a lot of extra use out of my garment with just one little trick!