Many years ago, before there were other alternatives, I installed a DN123 in an Atlas s4, it involved removing the circuit board, tracing out the runs, cutting them at certain points, then soldering the decoder wires to the correct spots while leaving enough wire attached to allow the decoder to be crammed into the cab. The NCE ATLS4-SR replaces the stock circuit board, it's only a little more work than a plug'n'play.
By the way, I am still running that S4, it pulls like a Clydesdale and runs smooth as silk.
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Mario1,
I'm guessing HO scale, and also that you meant a DZ123. If that's the case, and since you're asking for instructions, I wouldn't recommend using that decoder in that loco. It might be a difficult install project for a beginner.
A better choice would be the NCE ATLS4-SR, which Tony's also sells as their AtlasS1234. It's a drop-in decoder that includes "Golden Glow" LED's and the nylon screw required to isolate the lower brush. And of course, it comes with complete installation instructions.
HTH,Steve