That url is the info I used when I started changing the fonts. It's pretty concise and to the point, and gives exact info what you need to do without having to struggle through all the technical info behind the process or 100 posts. The only change you make on your reader is to add a new directory named "fonts" and to place the fonts you want to use in that directory. That can't possibly break anything on your reader.
Just make sure the info you add to the Extra CSS box names the fonts you transferred to the reader exactly and it should work for most books.
When it won't work (but still will *not* break anything), is when just changing the Extra CSS isn't enough to get rid of all the font calls from the book you're trying to change. How I get around that easily is to right click the title in the Calibre Library window. Select Tweak ePub, then click on Explode ePub. That will bring up an explorer window with all the various files included in the book. Open the file that ends in .css in whatever editor you prefer (I prefer PSPad,
have used it for years and love it, and it's free). Search for any reference to a font type, usually listed as "font-family" and change the font they specify to the font you want instead. It might be listed in quite a few locations within the file, search for them all and replace it. Such as:
In file named stylesheet1.css, find:
font-family: "Times New Roman";
I changed that line to read:
The file might be named a bit differently but it will always end in .css.
Save the file with your changes and close PSPad, then click on "Rebuild ePub" from the Calibre small window still open. And you're done. Upload the book to your Sony reader and enjoy your new font.
Edit: Be sure to always have quotes around the font name, or else the reader will still load the default. I was going crazy trying to figure out why one book still always displayed the default Sony font and realized I left the quotes off one of the dozen or so font calls in the .css file. Added the quotes and it finally worked.