Plugin way: (Heavy tldr;)
- Download the latest copy of Sourcemod for Windows and extract specifically the scripting folder in it.
- Download the tf2items.inc file and place it into the includes folder found inside of your scripting folder.
- Make a new file where compile.exe is titled 'my-weapon-changes.sp' or whatever it doesn't really matter. Just make sure there's no spaces, capital letters or weird characters in it and it has to be an .sp file.
- Look at example .sp files in the scripting directory to see how plugins are made but mainly look into how you can use TF2Items to make changes to weapons you want based on attributes, plenty of resources everywhere to see.
- Compile .sp file into the .smx file and load it into the server by placing it into the plugins directory to test it.
Easier route: (Also heavy tldr;)
- Install TF2Items and ignore the tf2items.inc file.
- Copy and paste the file 'tf2items.weapons.example.txt' in the configs folder it comes with and remove the '.example' part from it.
- Learn how to edit that file to manipulate weapons how you want since it does a lot of that kind of stuff already.
How to code plugins by Silvers is also available here:
https://forums.alliedmods.net/showthread.php?t=321089