Here is a drawing exercise of some shells. My wife has a large collection so I thought it would be an interesting subject to try.. The way I begin is to site size the drawing as best I can. The goal is to get the outside shape as accurate as possible before working on any details or shading. I line up different horizontal points (the height) then I use my pencil and my thumb to measure vertically(side to side). Then after as each point is checked and corrected connect the dots.
Next, I am just looking at positive and negative spaces. Black and white. Then I break it down to halftone, black and white.
3 hrs and the sketch is as far as I want to go. I should do this type of exercise every week. Training my eyes to see only the main shapes and tones is one of the best exercises I can do.
I may try to do this as an oil painting. The pinks and rose with the shadows would be a great challenge.