Follow along with the video below to see how to install our site as a web app on your home screen.
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For modeling, you need to read some XSI tutorials (although I'd recommend using the Maya tutorial edition or whatever its called, since you'll probably find Maya easier to learn). Learn how to make a simple model and export it into a file format that Source can use.
Both weapons and NPCs will need animations, although I think for NPCs you might be able to cheat a bit and use the existing rig and animations (as long as your NPC has the same proportions).
For the code you might be able to cheat again with both the weapon and the NPC by copying across code or simply copying across HL2 single-player code into your mod and then editing existing weapon code. There are tutorials on how to do this.
Then you need to learn how to use Hammer to make a map, a simple test room map can be learnt in a day. Then place your NPC in your map so you can see it, and your weapon in your mod so you can use it.