• Not pick up the phone
  
• Who are you?

• Pick up the phone
  
• Compensate/Run (does not matter, but there is a slight difference in scene afterwards)
  
• Going to cut off with ex

Follow the choices from Ending 2 except the last option.

• Tour with 2 guys
  
• Drink with Mongryong

• Not pick up the phone
  
• Oh yes honey
  
• Let's head out with Mongryong

Follow the choices for either Endings 3 or 4 except the last option.
Instead choose "Go to House Party" for 3 and "Sorry, I have an appointment" for 4. You're okay as long as you get to the house party scene.

• Go to House Party / Sorry, I have an appointment
  
• Don't fold

Follow either paths from Ending 5, except the last option.

• Fold
  
• I'll drink it

Follow either paths from Ending 6, except the last option.

• I'll accept someone who'll drink for me
  
• Amusement Park
  
• Call Jian

Follow either paths from Ending 7, except the last option.

• Do not call

Follow either paths from Ending 7, but choose a different place to go to the next day.

• I'll accept someone who'll drink for me
  
• Train Station
  
• Yes, go ahead

Follow the choices from Ending 9 except the last option.

• No, not yet

There's a short flashback scene shown here that you can only encounter beforehand if you choose "Sorry, I have an appointment" (see Ending 5). But missing it is not very important because it was still shown properly at this part of the drama.

• There is only 1 save slot, and when choosing to continue the game, it picks up from the start of the latest scene (though I had a playthrough when it picked up even before that. A glitch maybe?). Scenes can be quite long, so if you don't want to keep playbacking scenes, you can finish an ending before going to the Main Menu.
  
• I deem Endings 8 and 9 as Good Endings with the male leads, and Ending 10 as the True Ending - meaning as a drama this is where questions are resolved.

Feel free to suggest anything I can add in this guide or ask questions :D