With a scanner, silly.
And also, very carefully.
The problem with a book like R20 is both it's size and how willing you are to bend it. Me personally, I don't have a problem flexing the book open naturally, but I don't want to bend it back so hard that the pages get ruined. So my scans aren't anywhere near flawless. You'd have to gut the book from it's spine to do that. I can lift the lid off of my scanner all together, so with R20 I made sure it was off. With it's size, I can barely fit a whole page within the dimensions of my scanner.
If you look at one of my R20 scans like this one:
you can see that I obviously can't get the left hand side flat because I'm not folding the book back too hard. That's why there's some shadow there and not all of the characters make it in nicely. Not ideal, but best I could do all things considered. I made sure to hold that side of the book as steady as could be flush down on the scanner, while balancing the other part that I wasn't scanning in my other hand. Trying to balance the book so that it wouldn't slip or bend back any more than necessary. That way I'm not giving it any unnatural damage.
If you're not disassembling your copy, like the dokuganryu people did, that's about the only way I can explain how to do it without ruining it. Hope that answers your question.