Hello Astrologers. Natal chart by hand/The planet placement?

I have all the math done. I have calculated the cusps and planets etc. But now looking at a blank Natal Chart how do I know where the planets go. For instance I have cancer sun in 15 degrees in the 5th house. My 5th house is ruled by gemini in 27 degrees. How am I too know my sun goes in the 5th house? Thanks