Folks, if you set out to list all the possible ways of putting in 'French' characters, you've forgotten Unicode Hex Input. And one or two others.
The real point here is underlined by Cattus Thraex. Why does the OP need to enter these characters?
Is it for, say, an essay for History class on the age of Louis XIV? In that case, assuming she is using one of the US layouts, she doesn't really need to change anything. To spell correctly a few names of places and characters, using the traditional Mac way with the Option key ("Option", mon cher LexSchellings, not "Alt" -- let's talk Mac here) is fine.
However, if she's doing homework for a French class, and she needs to enter lengthy paragraphs, and she needs to do this more than once, then such a method is a major PITA, whose only virtue is that it's less of a PITA than any of the other methods suggested (Character Viewer indeed!) The only effective way to enter French text is to learn to type in French on a French-language keyboard layout. And there are basically three built-in options here:
(1) French (AZERTY -- there are actually two of them)
(2) Canadian French (CSA/ACNOR, based on QWERTY)
(3) Swiss French (based on QWERTZ)
She needs to choose the one closest to what she's accustomed to, and learn to touch type on it. If the OP is, as seems to be the uniform assumption, American or anglophone Canadian, hence familiar with QWERTY, then the CSA/ACNOR keyboard is best.
Finally, to be effective, she needs not only to enable the Input menu, but also to set a shortcut to cycle through enabled keyboard layouts, which is done in Keyboard Shortcuts.