IM in a last minute jam and could really use any help possible.

Im trying to format a song going through multiple scenes. It's a musical monatge that covers about 15 shots.

How do I write that there's background music?

Do I write the music in every scene?

Should I have formatted the montage in a special way which would make the music more clear?

Any info is apprciated.

Typically, you wouldn't mention music in a script. If the script is just for you, and not for agents or readers (which I'm guessing is the case) then you don't need to worry about it. There is no rule for music, so make one up

There are two reasons you would write a specific song in the script.
1. It is significant to the story. (Irony, etc.)
2. You feel it is needed for the reader to feel and understand the script.
Short of that, I wouldn't bother with overdosing the script on music. Let the reader feel it themselves.

Assuming this script is for you, only put things in the script that apply to the largest minority of your film crew. What parts of a song should play over what parts of a montage only applies to your editor, and your composer if we're talking original music. So write the screenplay for everyone, write a shot/music timeline or storyboard for your editor.