Are you bored of players' Invasions, blood stains, hints/messages everywhere, other players' annoying ghosts roaming out of nowhere? Here is the guide for you, the single-player gamer who prefers narrative and immersion to multiplayer online distracting events.
(Remember of course that offline may affect some multiplayer gameplay aspects like oaths to get the help from the other players or preventing you from invading others...)

Tutorial for Windows 10 (the concept is the same on older Windows versions):

• enter in Cortana/Start search: "Firewall" and run "Windows Firewall with Advanced Security"
  
• create a new rule for the Outbound connections (not Inbound!)
  
• click on "New rule" on the right and select the program "...\Dark Souls II Scholar of the First Sin\Game\DarkSoulsII.exe"
  
• choose to Block any kind of connection for Domain, Private, Public profiles in Advanced tab

Done. Now you can play alone in the DS2 world without immersion-breaking elements coming from the other players.
Remember you can disable/enable this rule whenever you want.

They'll still work fine, as well as Steam hours counter...
Also scripted invasions by CPU controlled NPCs will still work of course.
Steam chat and everything else will also work fine as usual while playing.

⚠ You won't get banned if you go back online of course. From a technical point of view it is exactly as if you were playing without Internet connection. That's the safest legit method.

That's why this is the best offline mode you can have.
Enjoy & Die!

(Small donations 💰 to buy and fix some more games are welcome from my profile page)