Once you've paid the $99 dollars to be a dev, this is what you do..
Make sure you register your device's UDID in the Developer Center before you install the iOS 7 beta.
Download the proper version of the beta for the iPhone you have. You should also go ahead and download the latest version of iTunes. Open the .dmg file and extract the iOS beta file - it will be a file that ends with ".ipsw."
Once you've downloaded all that, connect your iPhone to your computer and launch iTunes. Tap on the "iPhone" button in iTunes, which brings up your device's stats.
Hold down the "Option" button then hit the "Restore iPhone" button and you'll be given a file chooser window. Pick the iOS 7 beta file you just downloaded and you should be using iOS 7 in about 10 minutes. Maybe longer. I had some issues getting on the site due to high traffic.
Hope this helps if you do decide to become a dev!
Btw, I wouldn't download this current beta on a primary device. It's too buggy as of now IMHO.