Positioning key, fill, and hair light in Rembrandt-style two-light portrait

[LukeJJ]

I'm an amateur photographer trying to improve my indoor portrait work, and I'm struggling with portrait lighting techniques. I have a basic two-light setup with softboxes, but my results look flat and uninteresting compared to the dynamic, sculpted light I see in professional work. I'm particularly confused about how to properly use a hair light or accent light without it creating harsh highlights or lens flare. Can anyone explain the practical steps for positioning a key light and fill light to create Rembrandt or loop lighting patterns on a subject's face, and what are the common mistakes beginners make when starting with off-camera flash?