Here is a tutorial I get requested a lot, how to create a totally black background. No green screen required.
Here is the link I talked about in the video for using a grid for letterboxing.
I got the following message from OggOrbisMedia which I will have to try:
Instead of adjusting the level yourself use the color picker for the black level and select your not so black background. Then adjust the grey and white levels to your liking for your subjects exposure. That way you won’t risk making things too contrasty and compressed by bumping up the blacks more than you need to, and you don’t have to fiddle with it to get it right.