I taught myself a couple of tricks:
1. I mark a dot at the starting point by nesting the point of a very sharp pencil against the seam at the fold.
2. (Alternatively, with the piece opened flat, you can mark the seam line in all directions, making a crossing of lines at the intersections.)
3. I stitch from the fold (or you could call this the 'join or joint') to the edge rather than the other way.
4. Hand rotate the needle down into the dot or cross mark. Notice how it FEELS when the needle enters the fabric.....if you are off the mark and the needle goes thru 4 layers of fabrics, it will feel different than it will when going thru the 2 layers of fabric that you want it to. Once you have experienced that sensation, and know that the needle is just brushing the fold - not entering it, it will become easier to make these Y-seams more quickly.
Jan in VA