How to type on a sign?

I used to know how to type on a sign, but either my brain is a sieve (likely) or it's broken? I can't remember the steps. I have placed a sign, and I can see the pencil cursor. I have tried left mouse click, right click, middle, but I can't seem to get the typing dialogue window come up. What's the trick?

Ah! Thanks so much. Maybe some "tool tips" hints would be helpful in this game.