To simplify Jon's answer, you need to have a directory in the root of your Kobo's exposed storage called fonts (and yes, it is case sensitive). Inside that directory, you will place the font files. For Bookerly, the fontfile names would be:
Code:
Regular Bookerly-Regular.ttf Bookerly.ttf
Italic Bookerly-Italic.ttf Bookerlyi.ttf
Bold Bookerly-Bold.ttf Bookerlyb.ttf
Bold-Italic Bookerly-BoldItalic.ttf Bookerlyz.ttf
While Kobo allows both naming conventions, I prefer the more verbose version in the first column.
As for the Panose issue with the Bookerly font, I've attached two images of the old changes to the Panose weight from medium to book. As near as I can tell, with the current version of Bookerly extracted from recent Kindle firmware, the font file names are correct and the Panose weight does not seem to make any difference. I've attached images of my test ePub with Bookerly as the font without changes to the Panose weight (first image is a kepub, second image is an epub).