When you finish the two halves of the block, then you need to press your center seams. One side going one way and then the other side in the opposite direction, then there are opposing seams that nestle together when you place them together to sew them. This makes sewing them much easier. Put the seam on top that will push the other one into the seam line. The one seam is pushed into the other while sewing and it is almost fool-proof.