The FAQ How do I add a signature to my post? says: To add a signature to a post you must first create one via your User Control Panel.

I cannot discover how to create one via my user control panel. Forgive me if it's blindingly obvious, or if there's some better place to
ask the question.

The User Control Panel is up on top near the banner.
Click on that, then on Profile, and edit signature.

Also when creating a post you have to make sure the signature is visible, I don't remember what the default is.

Thank you Bríd - I'd already got as far as User Control Panel -> Profile, but there is no Edit signature option there: just Edit profile,
Edit avatar and Edit account settings. And I don't find a signature option under any of those. Do you see Edit signature right there
on your Profile page?

Some settings aren't available to new members, not until they've done ten posts. That could be one of them. Breandán will know.