The principle is to have a cylinder or ball or other shape that can be slid under the immediate area to be pressed, and to be of a shape that will fit the work snugly, and especially for curves, darts, and any tight corner or hard to reach spot .
The shape, or roll is stuffed with cotton filling until rock hard and then covered with a tight cover of fine linen, cotton or wool -–importantly, this cloth must not have any textured ridges or patterns that would imprint during ironing.
It’s easy to make your own for a particular job or a signature design shape.