I've only planted bluebonnets in a flower bed, so I don't have any personal experience. But the following links to the Wildflower center might help you:

For ground preparation:
http://www.wildflower.org/?nd=grd_articles&view=full&key=18

For bluebonnets:
http://www.wildflower.org/?nd=articles_wf&view=full&key=17

I have heard that one way to do it is to use Roundup to kill all the weeds and after a few weeks spread the seeds.

To get bluebonnet seeds to germinate, I've had good experience nipping each seed at the end OPPOSITE of the pointy end with nail clippers before planting. If you are doing it on a large scale, that would take too much time. I've also heard that you can throw the seeds in a blender and turn it on quick. You might ruin a few seeds, but it's obviously much quicker.

The other thing is NOT to mow the bluebonnets until its seeds has popped and spread. This is June in Houston and may be later further north. So the area might not look so good as the plants are turning brown and you are waiting for the seeds to ripen, but that's necessary.