So back to collars. This blouse was originally meant to be the muslin for a better blouse to come later. So I used my stash of poly-cotton broadcloth and cheap, non-woven fusible interfacing (garbage interfacing). My first collar attempt was sewn with my garbage interfacing and it looked terrible. For my next collar attempt, I switched to a lightweight woven fusible. It made my cheap fabric look amazing. You can really see the difference in the photo.
You can see that the primary problem with the garbage interfacing is good adhesion. And no matter how carefully I followed the instructions, it still failed. You can see the buckling and separation in the photo below.
Before I actually stitched my collar on for the second time, I decided to study a RTW shirt in greater detail. More about this later.