Put the skates on and lace them up as normal. Take a few laps around the rink to see how sharp the skates are and determine how much they need to be dulled to feel "right."
Open the door from the rink to the bench and stand in that doorway. Stand with one foot on the rubber flooring in the bench and place the blade of the other skate on the plastic portion of the boards where the door is open. Make sure you are distributing your weight evenly on the foot that is on the plastic and run your foot back and forth over the plastic.
Repeat Step 2 with the other foot. Make sure the motion you use when dulling the blades is a straight motion, going back and forth parallel to the blade. If you press the blade into the plastic and try to push in or outward (like making a hockey stop), you risk dulling the blade too much, or possibly chipping the blade.
Skate another lap or two around the rink, stopping frequently to test the sharpness of the blades. If they are still too sharp, repeat Steps 2 and 3, followed by another lap or two around the rink until the blades reach the perfect sharpness.
Use a hand stone to dull the blade if the plastic on the boards isn't dulling the skate blade enough. The hand stone can also be used to remove scratches or chips that are missing in the blade.