I have built out a 737 sim and have been using P2ATC to get the sense of what ATC is all about - looking to get going on VATSIM. First, I should note somebody has put a lot of time into upgrading the site and procedures, because I went down this road a few years ago and I find things much easier to navigate and understand. I've signed up at VATSTAR and am starting the process - I'm also reading documents here - it appears that much of the first round of testing/certification process takes place w/ a fixed wing/smaller plane. I only have access to the 737-800 as that's what I've built out (I