Ideally the two threads should meet in the center. If one thread is being pulled to the top, reduce the top tension. If one thread is being pulled to the bottom, reduce the bobbin tension. It isn't scary to change tensions as long as you note the setting so you can put them back later. My bobbin case has a tiny screw on it. I can change the tension by loosening or tightening that tiny screw. Take a picture of the bobbin screw before you start adjusting it so you can put the screw back if it doesn't help.