First, you will need a C++ compiler. I recommend MicroSoft Visual C++ 6.0, but this is an expensive piece of software. Its up to you to get it.
You also need te Half-Life SDK. SDK stands for Software Development Kit. The Half-Life SDK contain all the source those used to build the client.dll and hl.dll. These files are the actual Engine of Half-Life. They contain enemy AI, how the hud works, everything. Get the HL SDK here:
http://dev.valve-erc.com/projects/hlsdk/documents/103378972982381500/attached/hl_sdk_v23_source.exeAfter installing, open up the hl.dll workspace in our C++ compiler. To open the hl.dll workspace, simply look for the file hl.dsp in the Single-Player Sourcedlls directory.
Then go to fileview. And now i noticed that i made a mistake in my previous post. You will have to make a complete new file for the Otis to work, including some other adjustments in existing files.
Ill explain that later, after you have done all the above.