Know this and repeat to no one outside of Slytherin: Only a Slytherin may hug Professor Snape.
You must catch him unaware while he is alone in the dungeons. You must come out of the dark from behind him and wrap your arms around his middle. You cannot make a sound. You cannot cry out or whisper pathetically behind his back.
Listen here, little Snake, and listen well. He will tense and your heart will beat harder than you've ever felt it go. His hand will reach down to your own tangled ones, and you will fear for your life.
And yet, he will not hurt you, little one. He will recognize you as one of his own and he will treat you accordingly. His large hand will squeeze your small one gently, almost as though you were nothing but a precious ingredient of the highest quality.
And you may cry, but only if you do it silently.
When he removes his hand from your own, that's the signal. You must run like mad into the dark hallway that you came from as he yells out behind you.