Why Not Y?

Some folks shy away from tackling Y (or inset) seams but in truth they are no more difficult than sewing a mitred corner.  In the picture below you can see the Y shape formed at the intersection of the three seams. 

Whether your angle is 45°, 60° or even 120°, the method for stitching the Y seam is the same.

At the intersection of the three seams, the stitching stops the distance of your seam allowance from the end, in this case 1/4". You can see X's marked 1/4" from the edge of each section to be joined. I don't usually mark the X's but just eyeball the distance, however if you are new to doing this a mark helps.

You sew the first piece to the correct side up to the marked X.

At the X, lock your stitches and clip your threads.

The seam allowance at the end is left free.

Repeat to sew the second piece in place, again stopping at the centre X.

Place the side pieces (which will join to form the tail of the Y) right sides together and stitch from the X. Press the centre seam open, and the side seams down over that centre seam.

Pressing the centre seam open reduces the bulk of the seam allowance at the intersection allowing your work to lay perfectly flat.

Voila!  A perfect Y seam.