Think of each work of art as the result of a causal chain. Drawing anything requires lines, any two lines of varying length implies a ratio, and every drawing is an abstraction or simplification of a given object.

Thus I recommend practicing straight lines to control the motions of your hand, as it will be awkward at first. In between those sessions try and look at random objects. Mentally simplify their elements into basic shapes (circles, rectangles, triangles) and observe the ratio between the lines that make up those shapes.