Sometimes it is a matter of getting really acquainted with your machine. Your idea of taking the machine off the frame to practice bobbin threading is great. While you're at it, look at the rest of the machine and go through your instruction guide and just familiarize yourself with the machine. Videos are good, too. There probably will be a few hiccups along the way, but that is a part of the learning curve. Bet there is an online group of longarmers who have the same machine. Also look for online support from the Bernina company. Enjoy.