[Ripplinger]

It's really very simple to do. You need to include the font that you want, and usually in 4 styles (normal, italic, bold, bold italic). Place the fonts in a subdirectory named "fonts" in the root of the Reader partition, so as an example for the font in my screenshot above, the path to my fonts will be:
Reader\fonts\LexiaDaMa.otf, LexiaDaMa-Bold.otf, LexiaDaMa-Italic.otf, and LexiaDaMa-BoldItalic.otf

You can use either otf or ttf fonts, both work equally as well. Then you create a stylesheet that you place in Reader\database\system\PRSPlus\epub\LexiaDaMa.css. You can name it anything you like to identify your font in use. Here's a sample of my stylesheet for my favorite font that you can use as a template, just replace the name of the font with your exact name of the font you want to use. Note that you need to be particular about using caps if the font name is in caps, it is case sensitive.

Spoiler:

@font-face {
font-family: "LexiaDaMa";
font-weight: normal;
font-style: normal;
src: url(res:///Data/fonts/LexiaDaMa.otf);
}

@font-face {
font-family: "LexiaDaMa";
font-weight: bold;
font-style: normal;
src: url(res:///Data/fonts/LexiaDaMa-Bold.otf);
}

@font-face {
font-family: "LexiaDaMa";
font-weight: normal;
font-style: italic;
src: url(res:///Data/fonts/LexiaDaMa-Italic.otf);
}

@font-face {
font-family: "LexiaDaMa";
font-weight: bold;
font-style: italic;
src: url(res:///Data/fonts/LexiaDaMa-BoldItalic.otf);
}

body {
font-family: "LexiaDaMa";
}

p {
font-size: 1.2em !important;
text-align: justify !important;
line-height: 100% !important;
margin-top: 0.5em !important;
margin-bottom: 0.5em !important;
}

html {
font-family: LexiaDaMa !important;
}

.mainBody {
font-family: LexiaDaMa !important;
}

.calibre {
font-family: LexiaDaMa !important;
}

Some use much lengthier stylesheets trying to change every possible style called, but if you ever opened up an epub, you couldn't possibly come up with every single name that someone might use. I also found if the above doesn't change the font, it's usually embedded in the epub and it's best if you change the stylesheet within the epub itself to get rid of the specified fonts. You can do that with the Tweak option in Calibre and explode book, edit the stylesheet.css you find there, save it and rebuild book. Or the Modify ePub plugin in Calibre will often work. Or open the epub in Sigil and get rid of the specific font-family name references.

Once your FontName.css stylesheet in on the reader and the 4 font files, then under PRS+ Settings on the reader, select Book Viewer, User EPUB Style (CSS File), and select your new stylesheet there. You can add as many different fonts and stylesheets as you want to test and change them the same way. Go back to your book and you have your new font showing.

If you have any problems, post back, you'll get a lot of help. And I'll bet anything after you've explored all the various options in PRS+ that you'll install it on your 650 also.