ELI5: A stealth kill

Hi

I was looking in the book and can't seem to work out how I would for example Sneak up behind someone Thief style and bonk him on the head or Sniper someone from a mile away.

The only thing I found was surprise tests which seem stupid as there is no way if you randomly shoot someone walking down the street they would know its coming, and then I found the rules for melee that say you just auto hit and roll damage.

Please explain to me how taking someone out stealthy works mechanics wise for both close-quaters and ranged if possible.

Thanks