Disregard step four....you don't need to do anything with the barrel.
The trigger guard is just a friction fit, as you're shouldering the weapon (for orientation), the back part comes out...if you pull slightly, it will give way a little. I find it's easy to pry it out with a screwdriver from there.
Putting the rail on straight is tricky, when I had one I assembled the mount, but did not tighten it all the way, and then I used a level.