My first Marfy patterns is finished. I'm reasonably happy with the result of this pattern. The blouse is not quite my style, but I have a better idea of the size, and now I'm more confident to use a Marfy pattern, that's the happy me. A full review is on PatternReview.
As this is a simple blouse, I had no problems with the lack of instructions, and for most sewing issues I think there are enough books or other information (Internet, PatternReview) to be found to solve construction problems. So I'll use Marfy patterns more.
A little info on how I made the sleeve slit.
1. Fuse a little fusible interfacing to the end of the slit, then cut the slit
2. With right sides together stitch a length of bias cut fabric to the slit, folding the sleeve as in the picture, and take care not to stitch the fold. I find it helpfull to make the bias strip a bit wider at this first step.
3. Press the bias strip towards the slit, cut it to a smaller strip (twice the finished width) fold and pin to the slit.
4. Topstitch the slit, this is now finished.
Here a picture of finished sleeve slit with cuff.