There are two types of kimono: "Women's Kimono Collar A," which is a modern kimono that does not show a short collar, and "Women's Kimono Collar B," which is a retro and casual kimono that makes the half-collar look larger.
Load this "collar" material on the same layer where you first loaded the drawing doll on the canvas and adjust its position.
In the case of CLIP STUDIO Ver.2.2 or later versions, you can form a parent-child relationship with the body of the drawing doll as the parent and the collar as the child.
If you have a parent-child relationship, the collar will move with you when you move your body, so you don't have to adjust the position after applying the pose.
To form a parent-child relationship, in the "object list" of "sub tool detail", drag the object you want to make a child and overlay it on the object you want to make a parent. It's a little difficult to drag on the iPad, but it may be a good idea to do it with a long finger press instead of the Apple Pencil.
After making it a parent and child, select the child's object and set the "Attachment destination part".
In the case of this "collar", set it to "torso", and then align the collar with the doll.
After that, even if you let the doll pose or apply the pose, the position of the collar will follow the neck properly.
The collar of the kimono is roughly aligned like this.
The scale is generally based on realism and matches the body shape of the drawing doll, but the current CLIP STUDIO 3D can be individually sized for each object, so it can be adapted to the drawing doll with a customize body shape to some extent.
Please note that the length of the collar of each material is the minimum length so as not to interfere too much with the "torso", so the end of this object is not intended to be the "hanging collar line".
I think that the "women's kimono collar AB" can be the line of the hanging collar around the end of this object, but I think the other position is a little too high.