A block-in is a light sketch of your whole composition before any details or shading are done to make sure it’s placed correctly on the page and that the proportions and distances are accurate.
You can see from the block-in above that this light sketch doesn’t incorporate any refined shading or details, but it gets all the elements of the composition placed and sized correctly. The proportions and distances are accurate: the girl’s head isn’t too small for her body, the focal point (her body and the couch) are placed in the middle of the paper, etc. It’s the same idea as lightly sketching in the letters on a poster.
The block-in above is a typical example of how most professional artists block in a drawing. They’ve honed their observation skills and, as you can see, can get everything well-placed on the page. But many beginning artists would find that still too advanced for their skills.
So here’s a straightforward tip that will enable you to lay the foundations of your drawing before you get too invested: use axis lines.
Draw a single vertical and horizontal axis line, placed exactly halfway between the edges of the drawing paper, giving you a center point. Do this on both your reference photograph as well. Using these axis lines and your center point as reference points, begin to draft your drawing by gauging the distances of your objects from those reference points. After you’ve checked your work and everything is placed correctly, you can erase the axis lines then and start to shade.
Drawing Basics: The Beauty of a Block-In