Here's a two hour study from life I did yesterday, in response to my cohort Noah's still life with a crinkly old onion from Awesome Horse Studios:
(click image to enlarge)
I feel like I don't take enough time to step back and consider the basics I learned in all my art foundations classes, so here's a little process step-by-step I put together for myself:
Step 1: "Taking Notes"
This is really just a rough sketch like any other, but because none of the lines here will end up showing in the finished piece, I like to think of it as note taking. This stage is crucial for me because it's how I work out the relationship of the different objects in space. This is where I think about problems like "how big are the onions compared to the bowl of the glass?" and "how wide is the swath of fabric between the two onions?"
Step 2: Setting up a Value Structure and Basic Color
At this stage, I’m not worried about perfect mark-making or blending, just setting up a good value structure and roughing in color. When I'm painting using color (as opposed to pencil or other monochromatic media), it’s important that I consider value and color at the same time, as they’re closely related; it's also much easier* to start painting with color than it is to "colorize" a monochrome value drawing.
*some will argue this is not the case with digital techniques like using Overlay or Color layers, but I take a traditional-ish approach to digital painting and I recommend trying to think in value and color simultaneously.
Step 3: Color!
This is the part where I lay down all the color I'm going to use in the painting. Once this is done, I no longer sample any new colors from the Color Picker; all of my hues are sampled or mixed from within the painting using the Eyedropper tool. For unity, I’ve sampled some of ALL of the colors into all of the surfaces. Glass has no color of its own, and white smooth fabric reflects the colors of objects around it, so there are subtle hints of yellows and reds in each.
Step 4: Edge and Surface Definition
Unlike with a line drawing, edges in a painting are created by the sharpness and contrast between two bodies of color. Here I've defined hard edges (like where the red edges of the pepper meet both the background and white fabric behind) by painting areas of color directly next to one another with no blending between them, and soft edges (like those in the folds of the fabric) by smoothly blending one plane of fabric color into another.
Step 5: Finishing Touches
Adding little details like the texture of the onion skin and the veggie hairs coming from the tops of them I save until the end. At this stage, mark-making is important, so I'm conscious about trying to follow the contours of the objects with my brushstrokes. As with most of my finished digital pieces, I use Levels to make sure the contrast of my final image shows up beautifully on screen, and I might save out a separate version for printing with no 100% black.
As with any practical skill, actually doing it will make all of the above make more sense than reading about it. Sometimes explaining art fundamentals feels a little like explaining how to ride a bike without ever letting the student get on one and try it. So get out there and practice. :)