Mostly the same way as you used to do with pull-down menus. Press alt to display tooltips with shortcuts over ribbon buttons. Then you do bunch of alt + [letter] to get the option you want. The underscored letter in traditional pull-down menus in my opinion was similarly non-obvious.
And I don't mean the underscore, that was for fast access in the menu. Most menus would say actual hotkey on the right side of the menu by default.
But even the underscore gives a default visual clue that you can do something with that letter, and you can then ask what. If you're new to computers, the concept of anyotkey may not be obvious to even ask about.
I can take that out from my list of Ribbon problems :)