Interfacing should be cut the same shape and size as what you need to reinforce. Here I’ve made a mockup of a basic collar shape. If you were doing a button placket, you’d make it the same shape as just the placket, not the whole shirt front, since it’s just the placket you need to reinforce. I know, obvious, but I remember being confused about this the first time I worked with interfacing.
After cutting your interfacing, place it on the wrong side of the fabric
Pin in place and then sew 1/8 inch along the edge. You’ll want such a close edge so that when you actually construct the finished piece, the stitching lines from the interfacing don’t show.
You can finish by machine- or catch-stitching the loose edge of the interfacing to the wrong side of the fabric, so it doesn’t just flop around. This is more necessary with a larger piece, to keep things in place as you continue to construct.
And that’s it. SUPER simple.