The material I used

• One BR 85 Loco, Märklin 33081, originally with a Delta decoder.
   
  
• One ESU D1 decoder, which has sound and can be run with both DCC and Märklin Motorola digital protocols. It has "load dependent cruise control" as a Märklin 6090 or Lenz LE130 (DCC) and it can be set for use of 14 or 28 speed steps, 28 of course only applicable if you run a DCC layout. It comes with a 23 mm speaker and a "cup" (sound chamber) as default.
   
  
• One 28 mm speaker, since I ordered one extra with the decoder. "The bigger the better" was the logic behind this... :-)
   
  
• The motor of a 6090 kit I had at home
   
  
• Some drops of acrylic "super-glue" to mount the decoder & speaker
   
  
• I also used a Dremel tool to mill some plastic
   
  

What I started with
You can here see the 33081 loco, the complete ESU D1 decoder (wrapped in plastic film), and the extra 28 mm speaker I had ordered.

The difference of the standard 23 mm speaker and the 28 mm one.

Phase one
Ok, I have the materials, you can see the 6090 kit at the top of the picture. The loco has a Delta decoder and a normal Märklin DCM motor, these will have to be ripped out totally. You can also see where I have planned to place the 28 mm speaker.

Stripped bare naked
As you see in this picture, I stripped the chassis of the old motor and the Delta decoder. Only the red pickup-shoe and brown chassis ground wires remain.

Stuff is mounted
I used acrylic glue to fix the decoder into the cavity of the boiler, and also to fix the speaker to the body. The reason for choosing the BR 85 Märklin model is primarily that it has space enough internally to house the decoder & speaker. Another good starter model to do your first conversion with the ESU decoder would be e g the Märklin BR 86 (3396, 33961) model which has a plastic housing with lots of space in it (and Telex couplers which the ESU can control)...  The ESU decoder actually takes a lot of space since it is longer than the Märklin standard format and this can cause problems in smaller locos...  On the other hand it is narrower and can be squeezed into other places than a decoder would normally go. As you also can see in the picture, the motor from the 6090 set has been installed in the chassis.

Speaker mounting
Here you can see the 28 mm speaker mounted to the body of the loco. If you look carefully you can see I have milled some plastic off to get the speaker high enough so it will not touch the chassis. It is recommended to have a sound chamber behind the speaker, ESU have these custom plastic cups you can add, but this was not possible in this case....  I will have to try to do some kind of sound chamber later... 

Programming the decoder
Now here we come to the tricky part.....  If you wish to run the decoder on a Märklin digital layout, you cannot change anything but the loco address if you "only" have a 6021 Control Unit...   The decoder can be programmed with the software that ESU sells with the "Programmer" kit which also is rather expensive...  You will then need a computer to be able to do it. It can also be programmed with other equipment that supports DCC CV byte decoder programming. Luckily I use the Intellibox (IB), so I have the opportunity to program the decoder in DCC CV byte mode...  But it is a rather tedious process and if you plan to get a lot of the ESU decoders, maybe the "Programmer" kit that ESU offers might be worth its price..... If you wish to have the CV's I set to get it to sound somewhat credible, please mail me, the "webmaster"..

Observations & conclusions made 
The day after I had programmed the decoder, I took it with me to a railroad friend so we could compare it with the Märklin Insider BR 10. The friend seemed to like the ESU very much, since it sounded quite a lot more "credible" than the BR 10. He also remarked that I should have made a NEM decoder socket wired to the motor and lights and plugged the decoder into that...  Of course he was totally right about this, since the decoder was glued into the boiler and the body and chassis of the loco are now connected by the decoder cables into eternity...  I didn't even think of it while making the installation since I am normally used to 6090x decoders with free wires... Next time I will have it in mind....  However, the sound is great if you get it right, and is worth the cost if you can afford the decoder and wish to have credible steam sound. What is not so great is that the "load dependent speed regulation" is not at all so smooth as the Märklin 6090x decoders, it's kind of "jerky", like the Lenz LE130 if you look closely. Seems like it is designed for "flywheel" motors.... But if you choose to use the default EMF regulation frequency of 20 kHz instead of the "normal" Märklin or Lenz 80-90 Hz EMF frequency, you have a *very* silent Märklin motor, which is rather amazing.....

What's next ?
Well, since there are cables to connect a sensor so you can synchronize the sound with the wheel turns, this is obviously the next step make it perfect. ESU gives an example with a reed-contact + a mini-magnet on a wheel to trigger the reed-contact as a way to do it, and this is probably the easiest way to do it. But why aren't these cheap parts included with the rather expensive decoder ??? You will have to find them yourself in the nearest electronics store.....