There are a few disadvantages to this. First, it won't work for all cars. I went through four cars before I found one that I could clip this onto. The one in the picture is a 2001 Mercury Grand Marquis. Second, it has no shock absorber at all, so you'll get a pretty shaky camera. Gratefully the Mercury Grand Marquis rides pretty smooth. Third, it may not be able to handle a heavy camera.
Also, be aware that having anything in between you and your windshield when you're driving may be illegal. Check out your local laws before you go crazy with it.
A clamp - Try to find one that's strong enough to hold on, but won't leave too much of an indentation on your dashboard. You'll need one that has a hole for hanging it on the wall. You can see that this isn't the first project mine's been used on.
One 1/4 - 20 bolt - Your hardware store will know what you mean when you say that. It's a 1/4" shaft, 20 threads per inch. The length is up to you. Shorter is more stable, but it may not be able to see over your hood. Obviously you can take your camera with you when you buy the bolt just to be sure.
Three 1/4-20 nuts - You can see exactly where they go in the image. One to hold the bolt in place, one to hold the camera in place, and one to lock the other nut in place.
One lock washer - To keep your bolt from spinning.
Alternative - You could use two nylock nuts instead of three nuts and a washer. They are a bit more expensive, but not much.
Assemble as shown in the picture, then clamp it on have some fun with it.
That's it. Couldn't be much simpler.