Seizing the spreaders is a simple concept: you tie a knot that will hold the shrouds in the tip of the spreader and prevent movement of the spreader on the shrouds!
This may sound easy enough, but each spreader tip is different, and your boat may look different from this tip. You need to look at your spreader tip and evaluate how you can attach the seizing line to the shroud to the tip without damaging any of the components. This spreader tip had holes drilled through next to the shallow notches to hold mousing wire. Mousing wire will prevent the shroud from jumping out of the notch but it won't prevent movement on the spreader tip.
By tying the spreader tip up with dyneema and taking advantage of the holes in the spreader tip, it is possible to hold the shrouds in place and prevent movement at the spreader tip. The basics are to tie the spreader tip to the shroud and then to the shroud, this will hold the shroud in the notch and keep anything from moving at the same time.
You can easily visualize the damage that movement of the spreader tip can cause by observing the severe chafe in the lower portion of this photo. The spreader tip rested there during the initial setup of the synthetic standing rigging. Once the major adjustments were taken care of, I climbed the mast and positioned the spreader so that it would bisect the angle to the shrouds. This results in a slight up-sweep of the spreader (which places the drain hole at the lowest point to keep water from collecting in the spreaders if they are mounted correctly). The spreader was tapped and pushed into its correct place, several inches higher than where it was originally sitting and then seized in that position.
By removing any movement between the spreader tip and the shrouds, it is possible to reduce the chafe that will occur to the shrouds. Any damage that does occur between the shrouds and the spreaders will be limited to the serviced area. This sacrificial layer will protect the structural stay within from chafe while still being easy to repair if the chafe ever becomes too severe.