Cut your pillow panels the same measurement as your insert. So if you have a 18" by 18" pillow, cut 2 at 18" by 18"....no extra for seam allowance. I have found that the inserts usually run smaller than stated and/or are fuller in the middle and not flat.
Then fold and press the panels in half and then quarters. Measuring 1/2" down from raw edge, draw a line to the press mark in the center and trim. This allows the pillow to look full and not pointy with ears. I will be using this REALLY soon to cover some pillows that have mysteriously been adorned with popsicle stains.
picture not to scale....just trying to illustrate the cut lines