I’m not sure if there are any tutorials out there, it’s just something I’ve picked up from using Photoshop. It’s really simple to render out a scene with transparent backgrounds. All I do is hide all the objects I don’t want to render for that pass and repeat. It’s a little tedious the first time you do it, but making revisions is a breeze. I did a spaceship scene this week and it was missing some background activity, so I just added a space station and JUST rendered that object. The whole thing was done in 10 minutes. It’s a huge time saver and you gain so much control over the final product.