can’t do one smaller thing, so certainly can’t do a bigger thing
can’t do an easy thing, so certainly can’t do a hard thing

(start with the lesser thing, then ‘let alone (something bigger)’)

I can’t remember my first kiss, let alone if it was any good.

We don’t have enough money for a hamburger, let alone steak.

He can hardly speak English, never mind Chinese.

I didn’t notice you had gone, never mind cared.

His cat can hardly walk down the stairs, much less catch a mouse.

The voters didn’t like his appearance, much less his policies.