any advice for renting a flat from private, the dos and donts?

Hi guys,

I am about to rent a 4 bedroom flat in barcelona straight from the owner.
The owner asks for 2 montsh deposit.

Was wondering if any of you could give me advice about renting from private, the dos and the donts.
And what document should I sign / ask / provide, as like in many countries, im sure theres a scam regarding this, and I dont want to lose my 2 months deposit which is ?2400.

Moreover, I am planning to rent the rest of 3 bedroom, again what documents should I ask from / provide to the prospective tenant?

Thanks