"l4d1 events" comes up with the correct page as the first result.
Searching for specific things in quotes or putting "site:forums.alliedmods.net" to search for results only from here.
Less words, more keywords. Once you know those pages, bookmark them.
Another useful site is
Valve Developer to reference and find out stuff about various entities, how maps work, VScripts etc.
It would also be wise to run the sm_dump* commands to find out various netprops/datamaps entities have. Use various Dev tools plugins to make plugin writing easier. All this stuff is listed in the TUT thread I wrote.
Many things are transferable between L4D1 and L4D2, the things that are not are the new additions to L4D2 (new infected, new models/sounds, VScripts, etc). Some things will behave differently as they were updated or changed in L4D2. That's something you have to figure out testing or comparing with both games.
The plugin you linked is specifically for L4D2 because its either not possible in L4D1 or maybe the author doesn't have the game or just didn't want to find the offsets. It's patching the games memory using Source Scramble, this is more advanced stuff so you would be jumping in the deep end trying to modify that for L4D1.
I got into SM by viewing other plugins and reading through the wiki pages several times for years after I started and viewing more details about certain functions/natives in the API or include files themselves. I began by modifying existing plugins, simple things at first and more complex stuff later on, and then writing simple plugins by looking through similar plugins or searching AM forums for snippets. From that expanding the scope of knowledge by trying new things, using things I've not used before or doing things no one else has done, that's what I find fun.
There is enough information to learn and get by, you have to put the time into reading through stuff, learning from examples and testing things, lots of debug printing to see the flow of forwards being called, or where things are triggered, if the values you're expecting are actually those values etc.
Eventually you'll know all the basics and have an understanding of the language, then you can move onto more advanced stuff like SDKCalls, detours or memory patching, which opens up a whole world of possibilities.
I would suggest viewing through my plugins and those from other established scripters within L4D1/2. A lot of mine have comments about why something is done the way it is, or bugs encountered that resulted in the methods used. If you want, PM me. I can send an archive with all my scripts that you could use to search through for keywords. I also found that helpful when I started, downloading lots of plugins and then searching through those scripts for keywords on stuff I was trying to do.
Good luck.
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