for me, consists of a minimum number of lines that form a rhythmic pattern across the canvas. Some artists use vine charcoal. I use a very small brush and a liquidy turpentine mixture of an earth tone. Some artists spend a great deal of time on this step, being certain that each form is in the right place and that sizes and measurements are fairly correct. I find that spending too much time on the block-in not only neutralizes my enthusiasm for the painting part, but also reduces the amount of freedom in painting that I have...which is optimum for me in the creative process. And so, I forgo lots of measuring and exactitude which allows for the freedom I crave. The bad news is that not everything is in the "right" place and not everything is the "right" size. Adjustments will need to be made. Things will stay more flexible, more fluid. And that is all right by me.