If you start at the narrow tip of the triangle, the angle and distance of the beginning of the seam are difficult to estimate.

Starting on the wider end is much easier. The start of the seam is clearly visible. At the end of the seam simply sew a few stitches past the point where different areas meet.

Note:

Press your seam carefully. Trim the seam allowance back, especially at the narrow end of the triangle.

Also note the tip Handling Teensy-Weensy Pattern Pieces.