When you install the autobuy extension you will get an icon in the chrome search bar. Click on that and it will show a popup with an option to book ticket.
Once you hit Book ticket it will open up the chrome options page where you need to enter the following details:
>>IRCTC Login Details
>>Train To and From Stations with the Train Number
>>Passenger details with other options
After that you can save your ticket. At the time of booking visit IRCTC website by following the book ticket link from AutoTatkal. It will automatically fill the details that you have provided in the form. Enter the captcha wherever it is necessary. Then you will get the confirmed tickets