This is a classic because most Spanish people think that the translation is "mouth to mouth". We do use the expression "mouth to mouth" but only when you are trying to save someone from drowning!
Example: She had to give him "mouth to mouth" resuscitation at the side of the pool to make him breathe again. The way to say "de boca a boca" (marketing) is "word of mouth". Example: His company was successful because everybody was talking about his innovative methods, it spread by "word of mouth".
I hope that helps! If you are interested in AVOIDING more Spanish mistakes then you can check out my book "365 consejos para mejorar tu inglés" on Amazon"
Soy el director de Daniel Smith + Partners. Tengo más de 15 años de experiencia enseñando inglés a españoles.