I've gone through the stickies and done a few searches.
I can't seem to find a guide for starting a shadow priest from the beginning.
I'm a level 10 priest now.
No you aren't missing anything. We do not have a guide about leveling a priest. I did write a Beginner Priest Guide on H2P. It isn't focused just on leveling a shadow priest but there might be some useful information for you there.
If you have any specific questions though some people here might be able to help you.
Are you only wanting a guide that's on MMO-Champion? Or do you mean you can't find one anywhere.
If you mean on MMO-Champion, there may not be one. As much as the site has evolved to cover the whole game, a lot of the guides are still centered around endgame/new content, and as such are more gearing/mechanics changes that reflect the most current patch or expansion.
If you mean everywhere, well, I don't know what to tell you because they're out there. You might have to look somewhat hard for one that's nice and updated, but it shouldn't be that difficult to find (or to just use a slightly out of date one, and compare against other sources).
I know WoW insider also used to do huge leveling arcs, broken down like 10 levels at a time, but I couldn't find a recent one. They may have been in Wrath.