Ok I have time so I'll be detailed.
1) Insert your first source CD into your iMac. Open iTunes and you will se the CD appear in the Source list on the left. If it hasn't begun already, start the ripping process by pressing the "Import CD" button in the bottom right corner of iTunes 7. It's in the top right for older versions of iTunes.
The CD will begin to import, this will take some time.
2) Once imported, repeat step (1) for all other CD's that have Tracks you want.
3) Go to you Library by clicking "Music" in the Library Source list you should see your Tracks from the CD's here.
4) Click the little plus-button in the bottom left corner. It create a new playlist. Name the playlist. Now drag the tracks from the library to this playlist.
5) Once you've dragged the tracks, make sure you haven't got more than 74 minutes of music (because of CD size limit) in the playlist (check at bottom) and organize the tracks in the order you want them on your CD (by drag&drop).
6)Insert a blank CD-R. In your playlist, hit the Burn to Disc button in the bottom right corner (same place where you found the import CD button). Now the disc will burn. Once done, eject and enjoy.