After the whitening process, please take off the trays and rinse your mouth thoroughly. Use your finger tips to get any residual gel on your teeth or on your gums. You can brush your teeth very lightly. If you leave any gel on your teeth, it will continue working and that may cause sensitivity.
You should also clean your trays for your next session.
Stay away from staining food and drinks for 24-48 hours. If you must consume some, brush your teeth immediately.