We all find our favorite ways of finishing projects. For me, the following method of knitting front borders is one of the best ways to prevent flaring (the border looks too long for the edge). Flaring is often caused by picking up too many stitches along the front edge when using the common, horizontal technique for making front borders.
Vertical Ribbed Band – begin by working the extra border stitches into the bottom edge of fronts. For my cardigan pictured above, the border is 8 stitches wide, so I added 8 to the number of stitches cast on, and worked the lower front edge. You may have to plan for a buttonhole in the border when knitting the bottom front edge. After the lower front edge is finished, I worked to the last 8 stitches of the first pattern row of left front. These stitches are placed on a holder to be finished later. Finish knitting left front.
For the right front, work across the first 8 stitches; place these stitches on a holder and finish the row. Note: The buttonholes are made on the right front for women, and the left front for men. When all the pieces are finished return to the held stitches and finish knitting the borders, to fit along the front edges. Leave the last border stitches on holder at neckline or bind off, to include in the neck band. Slightly stretch the borders, pin in place, then sew the borders to the front edges. I like using an overcast (whipstitch) seam; a flat seam without bulk. An overcast seam is typically done on the wrong side, but can be worked on the right side with a contrast colored yarn to make a decorative seam.
I hope you will use vertical ribbed band for your next cardigan. The result is a perfect finished edge with no flaring.