To use Google Flights to find and book cheap flights type in the destination you want to go to and then click the date line and Google Maps will open a calendar which displays the cheapest flight prices for every day of the month.
Also if you really want to find the really cheapest flights use Incognito mode when browsing and searching Google Flights for cheap flights.
When you find the cheapest flight through Google Flights you'll then be taken to a third party website or the airlines website to purchase the plane ticket where you'll book the flight.