For all the seams that meet in the Y - sew up to the 1/4" seam allowance but no further. Then backstitch.
I take Y-seams in two steps always sewing from the outside to the inside. This helps to prevent puckers in the fabric and ensures that your corner fabric doesn't stretch out. Pin your fabric together and mark 1/4 inch from the end. Sew up to but not past the pin (picture below). Stopping a thread or two before the pin is better than going slightly over. Backstitch. (You need to backstitch in the corner because there will be nothing preventing you seam from unravelling.)
Now you do the same thing for the other side. Remember to stop just before the pin and backstitch. If you notice from the picture below, I fold the fabric out of the way so that I'm not fighting the fabric while sewing.