Dressing sticks are generally a rough grit aluminum oxide bar that when sliced through with a diamond blade will "strip off" a layer of the soft metal on the rim and allow the new diamonds underneath to be exposed. This helps to extend the life and efficiency of the saw blade.
It's simple enough to use a dressing stick. The best way is simply to make thin slices through the stick with the saw blade. There are varying ideas as to how often to do this. Many people will make a single slice through a stick before they use their blade on every occasion. Others will wait until their blades are not cutting efficiently and will make numerous cuts through the stick to bring the blade back to a sharp cut. We've tried both and would be comfortable recommending both ways.
Once you have used a dressing stick on your blade, use a magic marker or permanent marker of some sort and draw an arrow on your blade in the direction that you cut with it. Make sure to always cut and dress the blade in the same direction after that. If you change direction on the blade, you will reduce it's life span by prematurely losing the embedded diamonds in the sintered matrix.