I made a fixture to hold th heads in a vise. That sounds pretty fancy, but it is a 2x2 block of wood with a screw hole in it. Then I the same size diameter and thread screw that will attach te head to the decoy to hold the head to the block.
Where the neck of the head touches the block, I glued a small scrap of sandpaper, rough side out. The screw goes through the center of the paper. When tightened down the sand paper keep the head from spinning.