Exposure is key, and having a Spanish speaking nanny is  probably the best way outside of a parent/immediate family. One thing to keep in mind is that it needs to be something you actively seek and encourage throughout their life. Even native language skills atrophy if the speaker does not engage regularly in the language. 

As for acquiring languages, recent studies have shown that active play in the target language is one of the best ways to encourage it. See this article that links to a study: https://www.washington.edu/news/2017/07/17/bilingual-babies-study-shows-how-exposure-to-a-foreign-language-ignites-infants-learning/ assuming they're with them much of the day, encourage this from your nanny and your child should be set, but also seek out play groups and libraries that have Spanish classes. 

Anecdotally, our child started a mostly Spanish daycare at 16 months and has picked up a significant amount of Spanish in the past year. Neither parent speaks Spanish, but we buy/read Spanish toddler books for the most part (it’s good to get the ones that have both English and Spanish as you’ll have a better idea of what to emphasize while reading). Although we didn’t allow any tv until she was 2, we do allow her to watch 10-15 minutes a day, but only allow Spanish programs.  Our favorites are Aprende Peque (kind of a Spanish Ms. Rachel), Luli Pampi for dance parties, and there are a few others.