Add the tracks you want to convert to MP3 to a new playlist in iTunes by creating a new playlist, then dragging the songs to the playlist.
Burn the selected playlist to a blank CD in iTunes.
Click Edit, Preferences, General, then Import Settings. Change "Import Using AAC" to "Import Using MP3." Under Setting, you can choose different bit rates, from low quality 128 kpbs to better quality 256 or 320.
Click "Import CD" in iTunes. This will reimport your songs to iTunes as MP3s, and you will now have two versions -- one in AAC and one in MP3.
Download a freeware program like Free MP3 WMA Converter or a pay program like Xilisoft (see Resources for links) and install it.
Open the program and click File, then Add files to add the songs you want to convert to MP3.
Select a destination folder to save the new songs to.
Click Encoder to choose MP3 as the output format and quality settings for the MP3s.
Click Convert to convert the files to MP3.