Downloading and using fonts

I have read and re-read instructions on downloading fonts on my MacBook Pro. I cannot figure out why, if I install them into the Computer Systems Font folder, they are not available for use. I don't understand the relationship of the Font app and the System font folder--why they show up in one place but not the other. Also, when I want to select a type to use, I cannot select the location of the font I want to use. What am I doing wrong. I am using Sierra. My MacBook Pro is three to four years old.