For the centres I open main seams and press very flat. At this stage trim off excess ears. Match 8 into twos, then fours. Pressing the seams open. Now I pin the centre making the seams match. Now sew from the centre and check if correct. If not only small amount to undo. When ready do other side. I trim the seam a little to make lay flat. Ina sheared people hitting with hammers to make them go flat. The less you fiddle with stretchy edges the best. I also spray starch all fabric after I fave sewn each section before putting all 8 together. It makes it stay put in the centre,