If your option is take Home Whitening System, you need to follow this steps.
Place a small drop of gel into every compartment of the tray for the teeth undergoing treatment.
Seal the tray with the gel, around the teeth.
Wipe away excess gel in mouth with a tissue or dry soft brush.
After treatment remove tray. Rinse tray and mouth with lukewarm water.
And then, brush your teeth.