How do you backlight the keyboard?

You don't.

The current smart keyboard folio cases for the 2018/2020 iPad Pros have NO keyboard backlighting.

Sorry.

The new Magic Keyboard folio case, which is coming out in May, will have a backlit keyboard and a built-in trackpad and “floating” , unlimited adjustment platform/mount for the iPad Pro.

iPad Keyboards - Apple

Brydge keyboards is releasing in April a standard laptop style keyboard case, for the 2018/2020 iPad Pro, that has a full featured, backlit keyboard and addition of a trackpad, which is much cheaper in price than Apple's own future offering, if you do not care to spend $300-350 for the Magic Keyboard.

The Brydge PRO+

https://www.brydge.com/products/brydge-pro-plus-for-ipad-pro

Definitely worth a look at!

Best of Luck to You!