After you downloaded one. Open the folder and extract it by right clicking in a black space and selecting extract.
The .tff file should now look like this after you have extracted it :
Right click it and copy. Or press CTRL + C.
Open your control panel. Click Appearances and Themes. Then select Fonts.
Now paste the font. Edit > Paste or press CTRL + P on your keyboard.
It should say installing. And then when the window closes, it means you have successfully installed the font.