To install a custom font directly in your theme create a fonts directory under your active theme directory and upload the font files there.
Then insert this code into the active style.css file:
@font-face {
font-family: 'YourFontName';
src: url('/wp-content/themes/<your active theme>/fonts/YourFontFile.ttf') format("truetype");
font-weight: 400;
font-style: normal;
}