The strips usually come with gel pre-applied on an adhesive side. This side clings to the teeth and keeps contact between the teeth and the gel, allowing for whitening to occur. The best way to apply them is to use scissors to shape the strip according to the contours of your teeth, allowing for maximum coverage while avoiding contact with the gums.
Teeth whitening strips should be applied daily for a maximum of thirty minutes each day. For the first few days you should monitor closely for any negative reactions and discontinue immediately if irritation and sensitivity occurs. You should get results within about two weeks.
You can ask your dentist to show you to best use teeth whitening strips to reduce the risk of harming your teeth.