Hey! Try to follow the form of the face when using lines for shading (in general, always follow the form of the object when shading—avoid shading randomly or in directions that go against the form). For example, on the cheeks (the side planes), you could use slightly curved or vertical straight lines, while for the upper part of the cheek, you could use vertical lines. Another method you could try is laying down a soft base for the shading first, and then continuing to shade on top of it using line work. Another important aspect is line quality. Try using thicker lines on the shadow side and where forms overlap. This will also help give more volume to your drawings. My heavy hand doesn’t help much, and some of my lines ended up too thick haha. Because of that, I’m also attaching an example from Ognjen Sporin so you can see how he suggests volume with his lines. Hope this helps!
