For starters, this will not include code, just the programs necessary to coding your own.

Program list:
MCP - [ Register or Signin to view external links. ]
Eclipse - [ Register or Signin to view external links. ]
Java SE
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If you've never even looked at the world of Java, then you'll need a little training.
You can easily find training with Google, but in-case you somehow don't know how to use Google. Here's some help. [ Register or Signin to view external links. ]

Follow that, and after you've learned SOME code. You should have a basic knowledge of the language. You're ready to de-compile your Engine.

First, open up where you placed your MCP folder, after that, find your '.minecraft' folder (you can find it by going to Run > %appdata%), you should see the .minecraft folder then.
If not, die.
Copy, COPY! Not drag, the 'bin' folder into the 'jars' file. (Which is in the MCP folder, not .minecraft).

After that, click on the 'decomplied.bat' file. Let that run. (That file is in the MCP folder).

After that, open Eclipse, when it asks you where you want it to load from, give it the file URL of the MCP eclipse folder.

After that, all of the files should be there.

Boom, you have a decomplied version of Minecraft.

You can see where all of the files are to edit, and the InGameGUI.java is where you can edit and add the GUI of your client.

That is all for now, this tutorial will be updated.