How can I use a newly installed font as an input source?
You can not simply "type in another font" system-wide, since it is each application's responsibility to include support for fonts on its own. Installing a new font merely gives you the possibility to use it in applications, for example a plain-text application will never be able to handle fonts at all.

If what you want is to change the system font itself, then the answer is here: Can I change the font systemwide (10.8.3)