Gimp: Removing Solid Backgrounds

I bookmarked a tutorial that has since disappeared from the web. Don't you hate it when that happens? Anyway, this is really a tutorial for myself because it is something I do occasionally, but not frequently. Because of this, I always miss at least one step. It's not an in-depth for beginners tutorial, just a fast reminder of the procedure that I use. Again, this is geared as a reminder for me and isn't necessarily for the entire web. I'm putting it here so I can find it easily.

This is only for solid backgrounds.