I usually fold everything in fourths to help pin point the middle. Before all this though I press the top and backing but try not to drag and iron. Just press. Then fold in half and again. In the center I pin, then smooth without stretching work my way out and pin as I go. I do one quarter section at a time like the upper right hand section, then lower left hand section. Then the other sections. There is a YouTube video on pin basting and I believe one by Katie Amelie on pin basting a large quilt on a small table. Actually, there are a lot of videos on pin basting.