Here's my suggestion: (Maybe it's a system thing, but ignore all the crazy rules for a minute)
1. Write up your idea. Make it as in-depth, magical, and to your tastes as you possibly can. Remember to not go too far too soon. It needs to be a joy to play from start to finish, but the finish is still a long way off, right?
2. Have people you respect read it. There is absolutely no need for these people to be gamers. Ask for ideas, potential problems, crazy (even zany) changes to learn just how good your necromancer could be.
3. Research the idea in general. There must be tons of different takes on Necromancers throughout history and literature. Steal whatever you like and make a note of anything with potential. You can always come back later and rethink it.
4. Think about it in terms of roleplaying. No, not rules. Instead, think about feasibility, problems which might come up in play, how the class effects other characters or players, etc. I'm guessing most DMs will not allow a necromancer as they envision the class as always evil. Another problem is how the whole WORLD will treat the character. They're a heretic. Clerics won't heal them, paladins will be dispatched to kill them, even druids see undeath as an enemy to their natural balance. Don't get discouraged however. Sometimes these problems can be overcome.
5. Lastly, write up the game rules. Given the weight game balance has in 3E, I would go straight to the experts in the two Rules Forums. Even a basic outline is a good start. As long as you present a detailed description, I would think you should get a proper rule build in the end.
To me this sounds like potentially too much work. I prefer just coming up with a few unique ideas and building up from there. Not that I don't attempt to create something as awesome in the end. But if my character dies, all that work would go straight down the toilet.
If you are using D20, I suggest finding a DM who will allow you to play a custom class with only 2 or 3 levels actually mapped out. That should give you some flexiblity to change the future course of your character class as you play.
edit: I just remembered. You and your DM will probably end up creating a lot of unique spells too. This can be as fun as actually playing the character.