Here's a quick tutorial for making y~seams.
I've paper pieced the blue and white units, ready to add the white square using the y~seam method.
Mark a 1/4" seam line on the wrong side of the white square.
Lay the marked square, right sides together on the left blue and white unit. Line up the edges and place a setting pin through the marked intersection (X) matching it with the seam of the blue and white unit underneath.
Starting from the outer edge, stitch along the drawn line and stop with a small back stitch where the drawn lines intersect.
Take the second blue and white unit and place right sides together over the white square
Turn the unit over and place a setting pin in the drawn intersection on the white square, matching the blue and white seam underneath.
Begin stitching the seam where the previous stitching line stopped. Make sure the seams underneath are laying flat, take a tiny back stitch and stitch all the way to end of the fabric
Fold the stitched unit right sides together like this and pin to keep the blue fabrics lined up and together.
Start stitching exactly where the last stitched line stopped, taking a small back stitch and sew the blue fabrics pieces together.
Open up the seams and press.