I am new to quilting and I absolutely CANNOT figure this out. I am trying to do the Sister's Choice block which consists of squares and half squares. My problem comes in when I sew the two half squares together. The half squares that are sewn together end up being smaller than the full squares and then I don't know how to attach the half squares to the full squares. This of course ends up not making my corner points match up. I've attached some photos to show you what I mean. Any advice would be greatly appreciated.

Thank you

this is what happens when I try to attach 2 rows together,,they don't match up

do I sew the 2 half squares to the full squares like this in the center?

the outcome....points way off.