I have put this together to help you get the harem ending, its pretty straight forward and I hope you enjoy the route :)

Apart from the achievements from the normal route in the story, I'm only mentioning two achievements to be achieved during this route:

"Three's a Crowd" and "Haremly ever after"

Here is the order of choices I have put together for the ending

These are the choices used from the very beginning, when starting a new game

• Try and flatter Mitsuki
  
• I don't believe in monogomy
  
• I wanted to see you Mitsuki
  
• Let Mitsuki share your bed
  
• Nobody else can see you dressed like this but me
  
• Save yourself while you still can

Shortly after, Serra will now appear at the park and attempt to take Lilum back to heaven

• Yes, you're super adorable
  
• I won't go down without a fight
  
• Try and flatter Serra
  
• Warn Serra of the unspeakable horrors of that omelette

The third option should now be available while at the theme park

• Look around the park together

- Achievement unlocked "Three's a Crowd"

• Invite Serra into bed with you
  
• I want you and Mitsuki to be safe
  
• Wait for Serra

- Continue the story until the end for the other gallery picture.
- Other achievement unlocked "Haremly Ending"

Gallery images for the harem ending. A third gallery picture will be available if you have decided to apply the 18+ patch

I have now made another guide on the other routes.
Heres a link for this guide. I hope it helps

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1314231168