The skirt was really easy. I just folded three pleats on paper and then added on the folded paper the waist-hip-curve (by just copying the curve from my template) after x centimeters according to the width of my upper back and front part. This is how the front skirt part looks like (has to be elongated to the desired length):
Then I added the muslin skirt to my muslin top and it looks really good. I just have to take care when sewing the pleats. I want the ones in the middle to match with the darts of the upper part. I also tried the upper front part with two darts but this didn’t look as good as with one dart. So I’ll stick to my first version.