How do I type accented characters in MacOS?

If you go to the International pane of System Preferences and turn on the Input menu (I’m not at my Mac now, so I can’t give you exact details), there will also be an option to show the Keyboard Viewer in the Input menu. Once you’ve done that, you should see a little flag icon in your menubar, from which you can turn on the Keyboard Viewer.

Once the keyboard viewer is up, you can hold down option and/or shift to see what the different option combos will result in. It also has some kind of visual indicator (I think the key cap is outlined) for the “dead keys”, i.e. accents that you follow with the letter that goes below them.

A couple examples

Opt+E then e will show é
Opt+U then u will show ü
Opt+I then i will show î
Opt+` then a will show à
Opt+N then n will show ñ
Opt+C will show ç
Opt+O will show ø
Opt+1 will show ¡
Opt+Shift+? will show ¿