It will soak up water just like a sponge (right away), so no need to soak it. It's already sterile also, so no need to do anything special to it. You don't want it sitting in water because it will remain to saturated, so just dunk it in water then lightly shake off the excess water, then place seed (or sprouts) in it. I have a water spray bottle with just water in it, that I mist the rockwool once in a while when it looks like its drying out. Once they have sprouted you will want to use a diluted (like 1/2 to 1/3 or even possibly 1/4) strength
nutrient solution in place of water. Although I would not spray it on with the spray bottle, it might burn the delicate early foliage.