How do you say pretty smile in Chinese?

Just by itself, it would be: 漂亮的微笑

Piào liang de wéi xiào

(Adj + noun)

However, if you want to compliment someone [using it in a sentence] and say that they have a very pretty/beautiful smile, you can either put the adjective after or before the noun (like English).

So you can either use 漂亮 and 微笑 like this:

你漂亮的微笑真令人驚嘆.

Nǐ piàoliang de wéixiào zhēn lìng rén jīngtàn

Your beautiful smile amazes people.

(Your beautiful smile makes people amazed = literal translation.)

Or in the more common way:

你的微笑真漂亮.

Nǐ de wéixiào zhēn piàoliang

Your smile is really beautiful.