First thing you should do is to find/create the file path for the custom folder:

C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\custom

This is a dummy path look of what the file path should look like. Once you do create this keep the folder open so you can drag and drop the custom stuff in
there.

The most popular place to download custom tf2 files is:

https://gamebanana.com/games/297

You can install a ton of custom stuff from custom hitsounds to models but BE WARNED! Official Valve Servers will have these custom files disabled! Which
will result in missing textures and static sounds played. You have been warned.

Most files you will download might be in the VPK file format. VPK files can just be dragged and placed into the custom folder. But in some cases the
custom content you download will be folders with files in side. Open the compressed file it could be .RAR or .ZIP and drag the folder into custom. DO
NOT OPEN THE FOLDER AND TAKE THE CONTENTS OUT OF IT!

That is a quick summary of installing custom files for tf2.

Of course if you need a quick no sound viewing of navigating to your specific file path watch this video: https://youtu.be/irlfKFf4uAs