Collars on shirts, blouses, dresses and jackets come in many styles. Basic types include a "stand" portion of the collar that lifts it above the neckline, and a "fall" portion that folds down. Collars vary from an all-fall collar, like the flat Peter Pan style, to all-stand style such as a mandarin collar. The basic steps for sewing any collar are very similar with variations for different styles.
Cut the pieces according to the pattern directions. With a stand-up collar you will often have just two collar pieces --- front and back --- and no back-neck facing piece. Cut the interfacing to match one side of the collar.
Sew a line of stitching along the lower seam line of each collar section. This is the edge that will be attached to the neckline of the garment and is often marked with notches to help match it with the neckline. This line of stitching is to stabilise the collar edge and is called "stay-stitching."
Iron the fusible interfacing to the under-collar section. This will be the piece of the collar that goes to the inside of the garment. With fabrics such as a heavy material, fuse the interfacing to the outer collar.
Place the right sides of the collar pieces together and stitch along the top and both sides, leaving the stay-stitched edge open. Trim the seam to about 1/2 inch and clip off the tip of the fabric at the corners. Make vertical clips in the fabric along the sewn seam about 1-inch apart, just short of the stitching.
Clip the material on the open edge of the under collar at the points where the collar will meet the shoulder seams. These are usually marked on the pattern with a circle. Clip it to the stay-stitching. Fold under the middle portion of the open edge between the two clips on the under collar and press.
Pin the collar to the neck edge, matching the dots on the collar to the shoulder seams of the garment. While there is no back-neck facing for the stand-up collar, there may be a facing along the opening to the garment. The facing will continue along the neck edge to the shoulder seams. Pin the facing over the collar.
Baste the collar in place with the facing, if any. Stitch the seam. Both the outer and under edges of the collar, the facing and the garment are sewn together to the dots at the shoulder seams. Along the back-neck portion, only the outer collar and the garment are sewn. Do not catch the turned-under edge of the under collar in the seam along the back edge.
Turn the facings to the inside, if necessary. This should cause the collar to stand upright. Press the seam up along the back neck edge and hand-stitch the folded edge of the under collar to the seam with a slip-stitch.