A method including: providing a portion of double cloth including a first layer joined to a second layer, the layers being joined by a two-dimensional distribution of joining threads obtained by weaving; detaching the first layer from the second layer along an edge zone of the double cloth, to define on the first layer a band in which the first layer is separated from the second layer; cutting the band of the first layer, so that the second layer exhibits a projecting strip which projects with respect to a free edge of the first layer; arranging a further portion of fabric in contact with the second layer; joining the further portion of fabric to the second layer by a joining line arranged along the projecting strip; after the step of joining, turning the further portion of fabric, to move the further portion of fabric away from the second layer.