Shizuku's route is the 2nd easiest to get onto. when you get choices on what to say answer with these options when you see them.

"I'll go with Shizuku."
"I'll go with Shizuku."
"I'm with Shizuku."
"You and I can do it. I know we can."
"Are you... okay?"
"That cosplay will look good on you, Shizuku."
"They look amazing on all three of you."
"Plus, Shizuku, you're a dear friend to us all."
"You can count on my help."
"What other choice is there? We have to fight."
"Certainly!"
"I had a lot of fun too."
"I need to be there for Shizuku!"
"Never mind me. Are you hurt?"
"l'l protect Shizuku with my life!"
"The two of us together have nothing to fear, Shizuku!"

Tohko's route is the easiest to get to due to being childhood friends. when you get choices on what to say answer with these options when you see them.

"I'll go with Tohko."
"I'll go with Tohko."
"I think Tohko's got the right idea."
"We save Rin, of course!"
"Thanks. That would be a huge help."
"Are you asking me out on a date?"
"Thanks, I appreciate that. I'll let you know."
"I'm honoured!"
"I think Tohko can handle herself just fine."
"Let's show her what two childhood friends can do!"
"...Thank you, Tohko."
"We will. Because it's you and me."
"We did it together."

Shion is the 3rd hardest route to get onto because she is introduced after Shizuku and Tohka's routes are pretty much started. when you get choices on what to say answer with these options when you see them.

"You just leave these guys to me!"
"I say we let Ms. Kasugai do her drug thang."
"Maybe we should ask their boss..."
"A babe like you can get away with anything."
"I believe you, Ms. Kagusai."
"And for you, Ms. Kasugai."
"Sorry to have gotten you mixed up in this."
"Doesn't matter, because we won't let it happen!"
"Grave Danger is my porn name!"
"I believe in you, Ms. Kasugai."
"Okay!"
"I made a promise to Shion that I intend to keep."

Rin's route is the hardest to get onto because it starts around 25% through the game. when you get choices on what to say answer with these options when you see them.

"We're late! We need glowsticks and fans, stat!"
"Wait! We strike after the concert. No sooner."
"Well, we did disrupt her concert..."
"BRB. Need to sell her on Rin's awesomeness."
"I forgot to get her autograph. Come on!"
"I say we let Ms. Kasugai do her drug thang."
"Are you hurt?"
"My heart overfloweth with thine words."
"You don't mess with Rin's Elite Guard!"
"Uh, because I'm in your fan club? Duh?!"
"You treat your fans pretty well, huh?"
"I think cutesy would be a good look for you."
"No, no, it looks utterly fantastic on you!"
"Are you...worried about me?!"
"I will protect you to my dying breath, Rin!"
"Let's make this a show to remember!"

Nana's end is a bonus, you can do her end along with one of the main females so you should work towards this too. To get it check your emails after every story quest as Nana will give you some side quests, make sure you do them as soon as you get them, there will be 5 Nana quests and 1 Pops quest that needs to be done. The names of the emails are E-MER-GEN-CY * SUM-MON, E-MER-GEN-CY * MIS-SION, Order of the Highest Priority , -=** Emergency Mission ** =-, Hey you guuuuys!, Lethally fatal wound.

After this you will get a final quest called "Sister's Bodyguard", while on this quest Nana will talk to some of her friends, you need to pick certain things to say to avoid failing this ending, say the following to achieve it.
"I'm Nana's brother."
"Calm down, Nana."
"This sounds like one big misunderstanding."
"Okay, let's all calm down."
"This is...so beautiful. So beautiful! *sniffle*"

And thats all! have fun stripping!