Sometimes, you will have a collar or armhole that requires finishing with a facing piece that is backed with fusible interfacing. For beginner's patterns, these steps generally follow a set procedure. Working with fusible interfacing (as opposed to finishing with bias tape or other method) gives a collar structure and form, but does require several steps to see through. Fortunately, this a fairly simple procedure, as long as you take each step carefully, you will have it down in no time.
The first part is, obviously, is to fuse the interfacing piece to the wrong side of your matching fabric piece. Follow the link above for more information on how to do this. Once you've backed your fabric piece with interfacing, put the RIGHT sides of the pieces together as indicated in the diagram, matching notches where applicable, and then stitch them together with a regular ⅝" (15mm) seam or whatever is indicated in the pattern. Press your seams open, and then turn the right side under ¼" on the interfacing side to finish the outside edge. Stitch it in place-- most patterns use a straight-stitch, I prefer a zig-zag for this particular application.
After you've gotten the stitching in, you want to "trim seams/clip curves." What this means is you trim down the allowance between the raw edge and your new seam, you want to do it at about a 45° bias (if you are able) to help ease the fold. Generally, I trim between ¼"-⅜" from the allowance. You don't want to trim right up to the edge of the seam, because that will make the next step more difficult than it needs to be. When you're done trimming the seam, you "clip the curves," which means clipping notches into the curved edges of your allowances. Don't break your seam when you cut the notches in! Get those notches as close to the seam as you're comfortable with but don't break that stitch! You want notches every inch or so, depending on the severity of the curve.
Once you've finished trimming the seam and clipping the curves, fold the interfacing under so that the interfacing itself is enclosed between the fabric facing side and the interior of the garment. Press it into place, rolling the seam just to the interior of the garment so that it's not visible to the outside.
understitch will help keep the interfacing piece in the right spot, without creating a visible topstitch around your collar edge which might roll forward and look funny.
Watch the below video if this instruction seems a little funny--I find the picture that comes with the pattern to be a little confusing.
stitching in the ditch " (which is a straight stitch through an existing seam), and second by tacking by hand along a curve. I would recommend stitching in the ditch first, and then tacking by hand as necessary. If you are making a garment that is a solid color, do not tack by hand unless your thread matches perfectly, otherwise, the stitch will stand out. But to tack by hand is just to make a couple of tiny little hand-stitches that will secure the collar into place.
Your facing piece is now in place!