Which stage of Agile (SCRUM) should we start creating automation tests?

A little background of me - I'm a manual tester for almost 2 years within an Agile environment using SCRUM (1-2 weeks sprints). So I'm wanting to introduce automation testing in my work using Selenium WebDriver (with Java).

My question is during when should I test the functionality manually and when should I convert them for automation testing?

I have been reading and getting different approach, such as:

1. When a new sprint is starting, convert the user stories to automated scripts from the previous sprint, OR;
2. Convert the user stories within the same sprint.

Any advice/s would be very much appreciated. Thank you in advance.