The following is the operation procedure in Blender version 2.92.
The images are in the attached file.


01.jpg	Start with all the faces of the skirt removed.

02.jpg	Select armature object.

03.jpg	Press the I key to invert the selection.

04.jpg	Press H to hide all but the armature object.

05.jpg	Select an armature object and then change the setting to display the bone names.

06.jpg	Find the spine bones in pose mode and select one of them.

07.jpg	Hold down the Shift key and press the S key to display the pie menu and execute 'Cursor to Selected'.
	Then, the 3D cursor moves to the position of the selected spine bone.

08.jpg	Select 00372:15:0 from the list on the right to switch to edit mode.
	Now there are no faces in the 00372:15:0.

09.jpg	Execute Add->Plane from the menu at the top.

10.jpg	The added face is too large, so press the S key and use the mouse to make it smaller.
	Use the mouse wheel to zoom in a little.

11.jpg	Execute Face->Triangulate Faces from the top menu to split the square face into two triangles.

12.jpg	Delete one of the two triangles.

13.jpg	Now 00372:15:0 is a mesh object with only one triangular polygon.

14.jpg	The triangular face is still too big, so shrink it smaller.
	And the position moves a little.


The display angle and magnification are different, but this is the same state as the previously posted image.
This time I moved the 3D cursor to the position of the spine bone and added a square polygon there, but there are many other ways to do it.
