Before doing anything else, locate your Team Fortress 2 folder. You will usually find it in Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf. Here, you should create a folder named custom. This is where our HUD files will be stored.
The next thing to do is to download the HUD you wish to use. Usually, they will come downloaded in a compressed .zip, .rar, .7z or another compression file.
Then, extract the contents of the compressed folder using WinRAR or another similar program.
Choose your desired destination for the folder to extract to, and then locate the decompressed folder. Inside, depending on how the HUD has been packaged, you will always find 2 folders named "Resource" and "Scripts". Some HUDs may come with other folders, such as "Materials".
Now, copy the folder that "Resource" and "Scripts" are located inside (not the Resource and Scripts folders themselves) and paste it into the tf/custom folder. If done correctly, your directory should look similar to this:
Make sure not to paste the HUD folder into the tf/ directory, like this
Or else the HUD will not load In the same vein, make sure you don't copy too many folders into the tf/custom directory, like this
As again, the HUD will not load. Now, load up your game. If done correctly, you'll be greeted by a nice, new HUD!