Well, "how to fall down" depends on how you fall. And it assumes that you have enough time to react and react meaningfully.

Generally, when I have gone down sideways, I have had time to react and I take the fall on something like my upper leg or upper arm rather then something like my hand with all those bones to break.

However, I have done two falls over the handlebars and in those cases it happened too fast to react and I was landing on my head in any event.