I think you are doing it right, but using the wrong terms!
interfacing is the stuff which supports/stiffens fabric, vilene is a well known brand.
A facing is the piece of fabric, usually 2-3 inches wide, cut to the same shape as the neck opening and seamed to the main garment then turned to the inside to give a clean finish.
A lot of facings are supported with iron-on interfacing aka vilene.