I don't think I would be best at giving a tutorial in customizing fonts. Everything I learned about customizing fonts I got from 2 threads:
1. Kindle
custom font thread
2. Kobo
modified font thread
I know "Blossom" and "JSWolf" are both active font modifiers and have much better depth on knowledge than I do.
The way I have done it so far on the Kobo is:
1. Choose which font I like (ones that look good on computer often look bad on e-reader). I have always liked caecilia and already own it, so I choose that one.
2. Find a ttf/otf copy of the chosen font family. You need regular, italic, bold, and bold italic. I have been told that
TTF fonts render better on the e-ink screen. It's usually easy to change from one to the other in a font program.
3. Because I like a heavier font to make reading easier, I choose to use semi-bold(medium) or bold as my base for regular and then use heavy for my bold fonts.
4. Remember, font renderers find the names confusing and need to be specifically named (there are instructions in
this post that tells you what to do about this for kobo readers)
5. Start with the regular font only and make changes to it and upload it to the ereader to test it. Since it's lots of experimentation, working with only regular saves you the trouble on changing all versions of the font each time. Once you are happy with the regular font changes, then you can change them all for the final version.
6. There are several programs to modify fonts. I am not an expert in choosing one. Sorry.
7. I make several weighted versions of the "regular" font to try. Eg: I changed the weight using 10 em, 20 em, 30 em, 40 em, 50 em, 60 em, 70 em, 80 em, 90 em, and 100 em. Then I tested each version on the ereader using an epub that does not have an embedded font and choose the variation I thought looked best at the standard size I read at (in my case 60 em)
8. I then looked at the size and decided that I wanted it to be a bit larger at default. To do this, I made 10 variations of the weighted font that I liked from step 7, starting with a scale of 101% and moving up to 110% (from glyph origin, not the middle of the glyph) and then tested each of these, choosing 105% as the best variation for my Kobo.
9. Finally, increasing weight and scale can cause some of the letters to squish together in certain combos - ie ri, rn, rm, tr, ru. If this bothers you, you can change the "kern" of the combos to separate those letters better (google kerning in whatever font program you want to use).
10. Once I was happy with the base regular font variations, I repeated them with italic, bold, and bold-italic, changed the names to match up properly, then saved as ttf font files. I added these to a "fonts" folder on my kobo as instructed in the kobo fonts thread above. After this, I disconnected and power cycled my kobo.
As an aside, I strip most of the embedded css from the epubs and set up the line height in the kobo_extras.css file as 1.2. This gives me as much screen real estate on the Aura with the footer showing the total number of pages read per book (not chapter) and removing the header. It's too bad I can't take a screenshot right from the kobo. If I get time, I will try a poorly substituted point and shoot photo of how things look on my Aura.