I just throw words and pictures at the page until it looks good.
But for real theres a lot that goes into it. A good understanding of gameplay mechanics and depending on the build a good understanding of the lore are just a few of the requirements. A keen eye for good artwork is important as well. Those are the most important elements I can think of.
You can sometimes get away with not having a good understanding of the lore. I personally tend to avoid incorporating lore in my builds because I know I'll mess up somewhere and get pounced on by all the lore fiends here.
The ability to inspire someone else to pick up a controller and try a character. It's as simple as that...
If you can write and illustrate something in a way that makes someone else on the other side of the world load a new character and get (another) 20 hours of great gameplay then I think there's something very special about that.
People often forget that they are supposed to be presenting a guide for other people to follow, in many ways you're not actually writing for yourself. So being a great communicator both in your words and visuals is really important.
Get some people posting builds just because they want to join in but have completely missed the point of what a character build actually is, 'Bloodworks' is saturated in them.
Personally I don't think it's as much work as a lot of people have made it out to be. A good builder just needs to know how to have fun and how to make a "pretty page". If you can come up with an idea that's fun, then you've done the hard part. Play it! See if your idea actually is fun or if it was just an idea that should've stayed just that - an idea. Put some glamour on it, paste it in the ning editor and make sure everything looks nice and is clearly explained. If I'm being honest, writing up a build and searching for art is all part of the fun of character building. Click the "Add Discussion" button and then be willing to take criticism and advice. Tada! The builder's job is done!