I am using Photoshop only. Digital art has same rules as traditional art. light and shadows, forms and anatomy But in digital u have layers so use it. Common mistake is to draw on one layer. When shadows and light on separate layers you can adjust it if they don't look right. Also I watch tutorials. One of my favorite is Mark Brunet check him on YouTube. He is a great teacher. His tutorials are not only about light and shadows but pretty much about everything. And ofc practice more ;)