Step 1: Fill in your departure and arrival city and departure and arrival dates. Choose how many people will be flying and click on the "Search +/- 1 Day".
Step 2: Look for the cheapest flight near your departure and arrival dates. In some cases you will have to depart a day earlier than planned or even a day later.
Step 3: Click on "Select"
Step 4: The cheapest flight will be listed first in the list of flights for that specific dates. Click on the link underneath the airline carrier showing the cheapest flight and click on the "Select" button at the appropriate timeframe you wish to fly.
Step 5: Fill in your details and continue with booking.
That is how you search and book a cheap flight.