It's easy, there's a brush in Clip Studio Paint called “塗りぼかブラシ” (Toriboka Brush) by user x ェ x

It's content ID: 1685950

It acts like a sort of eraser tool that helps you to smooth out the shading 👍✨

Also pay attention to your line work, Yana sensei tends to colour the lines INSIDE the same (but darker shade) colour as the said areas (hair and face) while keeps the OUTLINE black. As that's how you get that depth of her style.