Foundational portrait lighting for Rembrandt or split lighting with a single key

[JasonL]

I've been doing portrait photography as a hobby for a few years, mostly using natural light, but I've just invested in my first two off-camera speedlights and a softbox to have more control. I'm struggling to move beyond flat, evenly lit shots and want to create more dramatic or cinematic looks, particularly with a single light source. For photographers experienced with studio lighting, what are some foundational portrait lighting techniques I should practice first to understand shaping and controlling light? How do you determine the optimal placement and modifier for a key light to achieve specific moods like Rembrandt or split lighting, and what are the most common mistakes beginners make when transitioning from natural to artificial light that I should avoid?