Ok, here are the schematics to connect an Atari ST to an Amstrad (Schneider in Germany and some other countries) monitor; they were scanned by Matthias Arndt from the Atari scene -nice chap, him (:

   The book these schematics are taken from is in German but it should be pretty self-explaining.
A couple of hints:

• "Rot" - red
  
• "Gruen" (Gru"n) - green
  
• "Blau" - blue
  
• "Composite sync" - obviously the composite sync signal
  
• "Masse" - Ground
  
  

   Mathias said that some old STs (presumably not STFMs/STe's) lack the composite sync signal, so you'll have to merge the horizontal and vertical sync signals as shown in the scan. No big deal.

   There's also a schematic to connect an old simple comp sync monitor (all signals go to the same wire, which entails lower quality, like the RCA I guess), and another one to add a speaker connector for the audio signal. Have fun!