I heavily starch my fabric (Dip in a bowl of 3/4 starch & 1/4 water, let air dry, press, then cut pattern pieces).
I often glue baste tricky joins that must be exact.
I use a thin needle (size 11) and thin lintless thread (like aurifil 50/2).
I draw a thin line exactly corner to corner, and stitch just next to the line on the side I will flip over to make the half sq triangles.
I remain flexible whether to press seams open, to the dark side, or spin open