For the interfacing, I used white woven muslin. Interesting that the shirt collars I have taken apart for research purposes show they used two pieces of fusible interfacing. One was the exact size of the collar. The other one was fused to the first but trimmed to stay out of the seam allowances but caught in the top stitching. I am not a fan of fusibles, but my DH seems to like it on his purchased shirts, so next time, I am going to give it a try.
Here's the collar and collar stand attached to the shirt.
All that's left is the button holes and buttons.