This works for me for single iso saturn games (I don't have segacd so you have to tell me if it works)
Listen to the MP3 (or WAV) files
They should sound ok & not have lots of clicks, pops, etc... if they need editing, try AUDACITY, it's a free WAV editor that can handle MP3's (if you get the LAME codec with it).
Prepare your files
Convert all your MP3's to WAV format (I like LAME, but use anything you're confortable with) and rename them if you need to (they must be in alphanumerical ISO9660 sort order, so I rename them 01.wav, 02.wav, 03.wav, etc... to avoid any problems with tracks being burned in the wrong order).
Get CD-R burner device I.D. (for CDRECORD)
You need to know what the device I.D. of your burner is... use 'cdrecord -scanbus' to find out what device I.D. your CD-burner/reader uses (mine is 0,0,0), you'll need these numbers for CDRECORD.
Burn the game
As with any CD-Recording application, CDRECORD will work best if you don't have any other programs running while you are burning a CD-R.
Burn the WAV files using CDRECORD, use the #'s you got from scanbus in place of 0,0,0 and you can burn at any speed (not just 4x like in my example below).