Look at the shapes through a viewfinder. My viewfinder has marks both up and across dividing the space into quarters.
Hold a brush handle or ruler at arms length aiming at the shapes you have identified. Notice the length of the handle that the shape represents. Compare that to the next shape. It might be twice as long as the first shape. We are looking at relative shape sizes.
Referring to the viewfinder, the first shape might be one quarter of the height in the viewfinder. The second shape then might be one half the height of the area in the viewfinder. The first shape might start at the base of the are being viewed, and the second shape might begin at the end of the first shape. By this means you can stitch together the shapes being viewed. The same goes for horizontal shape measures and angles.
If you mark the canvas or sketch you are going to draw on into quarters, then you can transpose what you have measured onto the drawing of the shapes. Check the measurements to make sure they are relatively correct. Errors generally occur when the arm with the measuring stick (brush or ruler) is not held out straight, locked - in at a constant distance from the eye.
To get to the next step, use a mixed grey to fill in each of the drawn shapes with the right value (as in Rx1). Review Rx1. Remember to squint to see relative value and eliminate detail.
Make as many starts as possible to internalize this skill. After a while you will be able to eliminate many of the steps. Your "seeing eye" will allow this as it develops.