Ayin Hara – is it a real thing? Should we be concerned? How does it relate to Rosh Hashana and Tashlich?

How does Ayin Hara work, and what causes it?
Is there a way to proactively protect yourself from an Ayin Hara?
Is there a basis for the hand that people hang up for good luck, the red string or the blue eye?
Does it help to say “bli ayin hara” and Poo Poo Poo?
Are the services that are for hire to remove Ayin Hara legitimate, and how much do they cost?