Pick a mixture of both S and M options, in 7-10 situations.
Good Ending
Have the character's love meter maxed out along with having the M option meter maxed out too.
In order to unlock Heaven Scenarios
You must choose the correct two words (highlighted in blue) at the chapters 7-10 of each story. Ex. Dark, Maniac, Ecstasy. Doing so will unlock that story for the character located in the special section.
Normal Ending
Can be obtained by maxing out love meter and S options.
The rest of the Sakamaki Brothers
In order to unlock Reiji, Shu, and Subaru's route, you must complete Ayato, Kanato or Laito's route to the end.