An old New England puzzle

Here’s a puzzler to twist your brain a smidge.

Apparently this word puzzle used to hang in a New Hampshire lodge near a fireplace. And the words form a very important series of instructions. Here’s the puzzle.

If the BMT put more :
If the B . putting :
Never put more : over a - der
You'd be an * it

Okay, does your brain hurt yet?

You’re going to hate yourself when you see the answer.

At least give it a good hard try. Sound it out. Think about the fact that some of the characters might have a different description, based upon their use in other media.

Drag your cursor between the two braces below, and the answer will reveal itself in magic Inviso-Text.

[ If the BMT put more :
If the grate be empty (“if the ‘Great B’ MT”), put more coal on (“put more colon”)

If the B . putting :
If the grate be full, stop (“if the ‘Great B’ ‘full-stop’) putting coal on (“colon”)

Never put more : over a – der
Never put more coal on (“colon”) over a high fender (over a “hyphen”-der)

You’d be an * it
You’d be an ass to risk (“asterisk”) it.
]

What can I say? New England rural humor.