Having previously cut out the collar I know had a very simple job to do. Attach it. It really is fairly simple.
I first stitched along the pencil lines leaving the bottom seem open.
Then I realized that my collar wasn’t quiet symmetrical so I ripped out a small part of the seem and resewed. I’m a little anal that way.
And then when I was measuring against the existing collar on the shirt I realized it was a tad too long, so I fixed that too. It only takes a second. I also trimmed off the corners to reduce the bulk.
Then I turned the collar and ironed it, turning the open seem to the wrong side. (The side of the fabric that isn’t seen.) Again super easy.
I pinned the new collar to the old collar. Pinning is not something I do a lot of but occasionally I feel the need to be accurate.
I sewed the new collar to the old collar on the underside (The part no one sees because it’s under the collar.) then I turned the collar, Lined it up and sewed over the fold and then top stitched along the edges of the collar (about 1/4 inch from the edge) catching the old collar underneath.
And voila! A new collar.