Over time I've figured out a system of sketching and painting that works for me. It's probably similar to other artists and still different from some.
Select an inspiration image.
I usually scroll through Pinterest, magazines, or Instagram to see what pops out and add my own ideas to it.
With pencil sketch out the image as loosely and lightly as I can. Then tighten the lines a little
Add permanent ink
I outline the final sketch with a permenent pen and erase the other stray lines.
I lay down a light layer of water color. After that has dried I add shadows and let them dry. I repeat this until the layers I need are done.
I go in to some spots with colored pencil to deepen the shadows and add more contrast.
White pen & ink
I add a few bright highlights with my white Sharpie paint pen. Then use my permanent pen to fix spots in the outline they need it.
Sprinkle with glitter.
What is your illustration process?