First, you need to choose the media you wish to backup to. Typically, a standard CD is adequate for a single session of 5 minutes 10-20 tracks at 24 bit. Of course, this is a ballpark figure, and depends on how many takes and splices you have used. You can check to see how large your session is by going to your session's folder on your audio drive, and right-clicking and going to properties. This will display the size of the session.

Pro Tools keeps all of its files needed for your session in one convenient place. The folder (your project name) contains the session name .PTS file (with the tape reel icon), an audio files, fade files, and possibly a plug-ins files folder. Simply bring these into your CD burning software, or whatever backup method you chose, and save these files. If necessary to split files across two or more tapes/cd's/whatever, you can do this, but be careful to make sure you know which directory they were in so you can put them where they belong when you need to access the backup.

Recovering the backup is simply the reverse- go to your backup medium and drag and drop the session and files to your audio files folder, and everything should be happy. Here is a brief description of the directory structure (we will use Song1 as the name, for example):

[Audio drive]
[] (this is the folder you should save to your backup medium)
Song1.pts (reel icon)
[Audio files] (folder)
[Fade Files]
[Plug-in Files] (may or may not be there)