On KAYAK you can create one by signing in and going directly to Price Alerts in your account or you can look for the "Track Prices" button on the top left-hand side of flight results. If you're searching hotels, select "Set Price Alert" at the top right of the hotel results list. If you're on our app, you can get to Price Alerts using the app's navigation menu and follow instructions there.
Sometimes travel dates aren't set in stone. If your preferred travel dates have some wiggle room, flexible dates will show you flights up to 3 days before/after your preferred dates. That way, you can see if leaving a day or two earlier will find you a better deal. You can also select the flexible "weekend" or "month" search options to widen your search range and find the cheapest price that works for you.