We can't actually make Completely new planets, we can just string a bunch of recycled modules together and call it a planet, Module editing isn't all that hard, Doom Dealer's tut may be outdated, but its still awesome. Also, Fred's got a Module editor built into the Kotor Tool, which I enjoy using, just its a bit limited due to the maps it supports.
DoomDealers Tutorial:
Here (
http://lucasforums.com/showthread.php?t=130451)
I suggest Starting off with something easy, like a merchant Square, with a few merchants, and some simple encounters, then move on to really difficult things like scripting Cut scenes.
GSCCC also made a great Storyline Tutorial That can be easily applied to planets.
GSCCC Tutorial (
http://lucasforums.com/showthread.php?t=157396)
Also, depending on which version of Kotor Your doing the planet for, You may need Darth333 Whereami coordinates extractor (you only need this for Kotor 2.)
Good Luck1