Newton's Principia ignited the Scientific Revolution, but the work-sheets showing how he composed his masterpiece have been lost. Fortunately, he left behind enough clues that make it possible to give a plausible reconstruction how he did it. Surprisingly, such a reconstruction has not been attempted before. In the winter of 1679, Robert Hooke initiated a correspondence with Newton outlining the physics of planetary motion. But Hooke was unable to formulate his concepts in mathematical form, and afterwards Newton accomplished this formulation which allowed him to give a geometrical expression for the passage of time, thus laying the foundations for the Principia. On Dec 10, 1684, four months after a visit of Edmond Halley, Newton sent the first manuscripts for the Principia to the London Royal Society, which he had made "designedly abstruse to be understood only be able Mathematicians". This lack of clarity remains up to the present time. In this talk, I will show, however, that with a pencil and a ruler, and without any calculus, good approximations of orbits for central forces can be calculated graphically that also clarify the content of the Principia.