Sewing Tip: One of the best ways to keep folded darts securely aligned during the stitching process is to iron the darts' folded edge right after they are aligned and pinned.
Once the dart is folded and aligned such that the two outer dart lines are perfectly overlapping, insert a few pins parallel to the folded edge, through both dart lines to secure. Carefully iron the dart's fold edge to stabilize this alignment. Inserting the pins parallel to the edge will not only facilitate the ironing process, it will also ensure that the two dart legs are perfectly matching.
It is very important that in the ironing process, you don't iron past the the vanishing point(s) into the flat garment surface as this can leave permanent crease on the garment's right side. Iron from the dart widest intake stopping right at or before the vanishing point.
Follow the same logic when ironing a contour dart's fold line: Insert the pins through the dart stitch lines (dart legs) parallel to the fold, keeping them away form the hot plate in the ironing process. Again, do not iron past the two vanishing points, staying only within the body of the dart.