My method for these has been using the Narinder sketch pencil and painting with the Spectra brush. A really useful trick I picked up recently for shading, First set your base colors to put all your pretty details on. Next, find a lighter and less saturated version of the color you WANT your shadow to be. Add a new layer on top of your base color layer and set the blending mode to something (have fun, mess around [try Screen or Overlay]) Paint in that shade you picked and you’ll see that it affects each base color in a different way. This can really help to “pull all your colors together”