An excellent way to learn to sketch is by following the sketching project prompts available here. It is actually a list of ideas which when sketched upon in a systematic way claims to make a person learn sketching for 6 months. (By sketching they mean rough drawings here).
It might be a good idea to formulate one's own prompts in the same way if one wants to learn more precise drawing. But then again, it might interfere with the creative process; the urge that moves anyone to draw anything is really not something that can be determined months in advance. Since that urge is what lends honesty and integrity to the artwork, this pre-determined syllabus-based approach is probably not best suited for "proper" drawings. Its utility is limited to practice sketches.