Heya. I'm one of the newest members in the Map-making community, and I have issues that arent easily solved. for now though, I'll just ask one question.
Go to D:\Steam\steamapps\common\Team Fortress 2\tf\custom\assets or where-ever your hammer files are and place in the custom assets. If you don't have a custom or assets folder you can just create them. Generally the custom models should come in a Models folder aswell so keep them in that. If you want to have these models load in-game you need to do the same with your games files.
Wherever you’re getting your models from, they should come in one of two formats where it’s either packed in a .vpk file or contained in a root folder hosting the content. Drop the .vpk or root folder into “tf/custom” and it will be available to be in your map.
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