Hi Anne, it looks like you’re doing everything right judging by the screenshot. I think what’s going on here is that Blender’s cloth sim isn’t liking the fact that all of the pinned vertices are perfectly aligned along the X axis, and are being scaled in that direction as well. You could try offsetting the vertices in the shape key along the Y axis, which should help some. However, I’d recommend giving Blender 2.8 a try as the cloth sim got a fairly significant upgrade that helps it to wrinkle much more naturally and gets rid of those failed edge cases. Here’s what I got in that version while following your same steps:
