번역 - 영어-핀란드어 - Say, how to Talk to Girls?

I shouldn't think this would be chosen as a title of a book as it is.

We normally use "kerro" instead of "sano", however "sano" is alright. (Or use "sanohan" or "sanopa" or "kerropa" where -pa would be more encouraging or urging.)

Word order (which is quite free in Finnish) tells us what is important > I would put an emphasis to "tytöille" moving it earlier in the phrase. (puhutaan tytöille > tytöille puhutaan) Also possible to use "tyttöjen kanssa"...

My suggestion as a translation in general:
Kerropa miten tytöille puhutaan? (would leave the grammatical comma out in a title)
(And as a title, maybe even in a short form:
Miten tytöille puhutaan?)