What does "[expletive] it up" mean?

She meant that her directions were so simple you couldn't misunderstand them. To fuck something up means to get it very badly wrong (see FUBAR).

It's definitely not for use in formal situations, or for use around those with sensitive ears.

It's also not exclusively an Americanism, as I've heard it used in the UK and other English language countries, although I couldn't speculate as to the origin.

It means you can't get it wrong; it's idiot-proof.

This is coarse slang not suitable for formal or sensitive settings. A polite alternative is "you can't go wrong".

fuck something up (or fuck up): do something badly or ineptly.

It's vulgar slang, but not particularly an Americanism. It is synonymous to “screw something up”. In your particular case, you can't fail to find the freeway.