Holding down the key to get an accented letter is great, I didn't know about that. I was trying to type degrees F and degrees C, using the little circle for degrees. I see it in font book, and I had to copy and paste the character. I wanted to know what keystroke to use, maybe copy and paste is all there is. I finally found this, which helped OS X El Capitan: Use the Character Viewer
It says "Open the Character Viewer: In a document, choose Edit > Emoji & Symbols, or press Control-Command-Space."
Neither worked for me (using Microsoft Word, an old version, maybe that's why). Should it work in Word? Nor did it work in Mail.
However, this really did help: "To open Keyboard preferences, choose Apple menu > System Preferences, click Keyboard, then click Keyboard." Then you see a check box for "Show keyboard, emoji, & symbol viewers in menu bar". Voila!