The point of a facing is to finish a garment hole, such as the neck or arm hole, without trim or binding. It has the useful side effect of making the faced opening more resistant to stretching.
This page is going to talk about how to draft and sew a facing for a t-tunic specifically. For more modern garments the sewing steps are similar, but the drafting can be more easily accomplished using the original pattern pieces as a guide. The important thing to remember is that the inside edge of the facing piece must match the inside edge of the opening you are facing and the facing should have the same grain direction as the opening at all points. This means you might have to sew a facing out of several pieces for more modern garments. Here is a good guide for modern facings, in case this is all sounding very confusing.
Generally when I make a t-tunic I've got a vague pattern (another t-tunic) but I have to make up the neck opening by hand. I have tried binding the opening but it looks awful and is devilish hard. Really, the easiest and nicest way to finish it is to make a facing. Since all I've got is the piece I cut out of the neck, I can use that to draft the facing. If you cut out your neck in several stages to get it the right size, you can simply trace the neck opening onto some paper before sewing the t-tunic side seams.
The instructions for each step of the facing process are listed under the pictures linked on this page. They should be listed in order. Once you've drafted the facing, all that remains is to sew it to the outside of your garment, turn it to the inside, and sew around the finished edge to keep it on the inside (don't skip this last part, you'll regret it).
