I made these practice buttonholes on a scrap to illustrate the problem. #1 has a piece of interfacing between the silk (folded over like you would in the pattern). I was out of the sew in that I previously mentioned, so I used a lightweight piece of white cotton. #2 is just the 2 layers of silk. #1 is perfect, #2... well, if this happened to your blouse, good luck picking it out. And if you're a beginner, then how discouraging. Buttonholes can be frightening enough to a beginner, what with attachments to your machine, and digging out the instruction manual. And, yeah, I only know how to make them myself with the attachment, so if you don't even have one of those... I don't know what to tell you.
Interface, people! You will be happier, and your buttonholes will too. You will need to interface both back openings because the interfacing helps the blouse fabric support the buttons.
Fusible is self-explanatory, but here is a pic of how I interfaced my extensions:
Basically, I pressed on the foldline, and then butted the interfacing up to the fold, and using a whipstitch secured it to the extension. I machine sewed along the hemline on the edge, and then folded along that seam and made the hem as instructed. This helped because my cutting was a not quite as straight as I would have liked. I mentioned the slippy dippy silk previously I'm sure. Cutting silk (or any slippy fabric) can be a challenge.
My back view:
A couple of final notes: I put a snap up by the neckline. The buttons start rather low, and I would have changed the positioning, but I only had those 5, so just made do. Also, I'm not sure if you can see it, but the dark spot near the hem is a pin. I'm also going to add a snap there, otherwise, the blouse tends to gape open at the bottom, seemingly pointing out the wideness of my backside. My backside doesn't need any help with that!
I will definitely make this pattern again, and I love the stylings of all the Colette Patterns. The lack of interfacing for the back opening is really my biggest and most important critique, and I think it's very suitable for a beginner as long as the right fabric is chosen, and some attention is paid to the fit.