@MarshallSpinney
You put this question in the Canvas Catalog forum, but it seems like you're asking about how to do this from within Canvas. I'll give the answer for that method.
There are multiple ways to do this, but for general messages not associated with specific assignments, see this lesson: How do I send a message to a user in a course in the Inbox?
Basically, it comes down to these steps:
- Click on Inbox from the global navigation menu
- Click on the Compose icon (looks like a pencil in front of a square).
- Click in the Course box and select the course
- Click on the To box and navigate to Teachers and then select your teacher.
- Type a subject and message
- Click send
Note that some instructors may want you to actually email them as opposed to sending a message through Canvas. You would use an email client such as Outlook, Gmail, etc., to do that. Other instructors use email to refer to the Canvas inbox.