All signature files are now auto-created using the Social Info for Twitter, LinkedIn, Website, and Media links that you already include in your user bio. For example, if you had all four links, your signature file would look like this: Follow me on Twitter or LinkedIn. Check out my website or some of my other work here. Note that Facebook and Instagram links are not included at this time.

See below for an example of what your signature might look like:

To add social links, navigate to your Profile.

If there's anything else that you would like to be displayed on the article page, you can add it to your About section, as that shows at the bottom of each of your articles in the "About the Author" block!
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You have the ability to turn your signature file on or off at the post level using the toggle within the publish menu: