You'll have to be more specific.
Do you mean the purpose of the PBE or the process of installing/Signing up?
To sign up, the e-mail you received has a link at the bottom to sign up for the PBE (Yes, you can use your Live accounts info again)
Once you sign up, you will be re-directed to the download link for the PBE client. (Make sure you are downloading LOLPBE.zip)
Once downloaded, use Windows to unzip the file. (Right click LOLPBE.zip, "Unzip All...")
Once unzipped, open the folder, and double click lol.launcher (Not the admin one)
Atm, you won't be able to patch, but later on today (hopefully) it should start patching.
Hope that helped.