This is a simple technique for invisibly finishing arm holes or necklines using bias tape (seen above on the arm hole of my Pink Peplum Brocade Top. For the technique used to pipe the neckline, click here).
If you have double-fold bias tape, press it open so that it is single fold (top row). Take your single-fold bias tape and iron one of the folds open (bottom row).
Pin your bias tape along your arm hole such that the slight crease remaining from where you ironed open the bias tape is along your seam allowance (in the picture above, 5/8″). If you want to change the shape of your arm hole, this is a great opportunity to do so. Instead of pinning the bias tape a certain distance from your fabric edge, pin it such that the crease follows your desired finished edge.
Sew along the crease. Trim down and finish your edges. This can be done with a serger, but I often just pink my seams (as shown above). If you fabric doesn’t ravel, finishing the edges isn’t important, but it is imperative that you trim down the seam allowance so that it is less than the width of the bias tape.
A hint: Where your bias tape overlaps, fold over the edge of the bias tape that goes on the bottom for an automatically finished edge to your tape.
Finally, you fold the bias tape to the inside and iron it flat. The bias of the tape will allow it to stretch around the curve. You can machine sew the unattached side of the tape (leaving a visible seam on the right side of your top), pick stitch the tape in place (leaving barely visible pick stitches on the right side), or if you have underlined your top, you can hand sew it to just the underlining and it’s completely invisible on the right side. Pictured above is the anatomy of the finished armhole for a bodice with princess seams and underlining.