First learn basic modding. Learn how to read JSON and understand how it works. Planets are not necessarily hard, but they are involved.
http://community.playstarbound.com/threads/adding-biomes-101.101968/ Sayter posted a tutorial a while back, I'm pretty sure it's up-to-date. Assuming you meant biomes, since planets are just icons that take you to a ball of biomes.
It is still valid. The celestial.config file has not been changed much. The biggest change was in a recent nightly where they decreased the star density (Just as my "Space Edits" mod does), reduced the number of possible planets around any given star from 10 to 7 (Not including ancient gates), and made all possible satellites only "moons" instead. Sounds like a drastic change, but it shouldn't hurt that guide much if at all.