I made slits on the sleeves - opposite to the seam. I cut a strip, sewed it on the right side, then cut it as in the link, and turned to the wrong side. I basted it flat. You can see how I did the corners. I unbasted a bit and folded the corner along the center axis. Then I sewed the corner 45° to the seam. I cut the leftovers.
Then I turned it to the wrong side and finished basting. As at the neckline, I don't plan to sew this on the machine anymore..
This is shat the slit looks like now. I had been worried whether the slit was or wasn't placed right. I, personally, would have place it more to the front. But I saw it like this at the sample.
When one moves, it surprisingly works:) I'm lucky to have bought a new bra, this one seems to be too small for me after Christmas:-DDD