I prepare to attach these by simply touching them to the top of the slip (it is sticky and coats the underside evenly). Then it is just a matter of setting it in place and and it glues down in seconds. I make these a little larger diameter than the top of the opening (e.g. 2mm) in the plaster mold, producing a small overhang on the cast piece. During the time in the mold, the clay slurry creates a thickness against the plaster walls but also extends upward above the top of the mold (inside the spout). A while after pour-out I am able to make a cut and lift the spout away leaving an over-hang, over-height and thickness that all play to enable jiggering the rim to the exact contour I want.
A process in ceramic production where items are slip-cast first and then finished using a jigger wheel.