The pattern is written in us terms, However the hook size is in mm
You need to put a stitch marker at the last stitch of each round to mark the end of the round.
You may want to make the pillow case 0.5 inch smaller than the actually pillow insert. To make sure that your pillow case fits perfectly without any loose areas, pumps or gaps.
The diameter of the circles I used to trim my pompoms are 1.5” 4 cm