The process for just adding the screensaver hack is actually pretty simple, you just have to dig through that page to find it.
Go to that link to download the jailbreak file. The readme provided in that download tells you how to jailbreak your kindle, but basically you take the mp3 provided, put it in your kindle's music folder, play it on your kindle's mp3 player and click the 'Press to Jailbreak' button that shows up.
Then go to this link:
Download the screensaver hack at that link. What you'll then do is take the file called update_ simple_screensaver_1.0_install.bin and place it in the root folder of your kindle. Then go into your kindle's settings, press menu, and select 'Update Your Kindle'.
Now you can change your Kindle's screensavers. Read the README provided with the screensaver hack or the IMAGE_NAMING_RULES file in your kindle's new screensaver folder to see the requirements for images to be screensavers.