This quick tutorial goes over how to use the mixer brush tool to achieve smooth blending in photoshop. If you'd like to have even smoother results, you can always just paint your light and shadow tones with an airbrush instead, and blend those in. TBH, that's usually what I do with my own pieces because I find blending solid chunks can be really tedious and time-consuming. Anyway, if you'd like to follow or support the things I'm creating feel free to from any of the links on my Link Tree here: https://linktr.ee/jaeharuart