First and foremost: DO NOT SKIP CUTSCENES! Unfortunately if you skip a cutscene that has a CG Scene you haven't viewed yet, you will not get credit for it and have to replay the event.
You can view what you've unlocked so far in the Main Menu / Extras / Character Art. There are a total of 18 CG Scenes required for the achievement, which is more than the Steam guide indicates as this game got DLC at some point that increased the number.
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01. Philia at the Amusement Park - unlocks when you take her on a date there
02. Philia at the Botanical Garden - unlocks when you take her on a date there
03. Mira at the Amusement Park - unlocks when you take her on a date there
04. Mira at the Botanical Garden - unlocks when you take her on a date there
05. Philia tasting shaved ice - unlocks when you give her shaved ice for the first time
06. Philia kissing you - unlocks when you kiss her for the first time
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07. Mira kissing you - unlocks when you kiss her for the first time
08. Philia in a car - unlocks early in the store when she comes to visit you for a second time
09. Philia in a bikini - unlocks after you buy her a bikini and take her to the beach
10. Mira in a bikini - unlocks when you take her to the beach
11. Philia with her arms crossed - unlocks after you get maximum favourability with her and then choose the ending where you return to her world with her. The Steam guide also says you have to keep going on dates and kissing her until she says "is kissing enough?", so I did this to be safe
12. Mira on her hands and knees - unlocked after you get maximum favourability with her and then choose the "Seolhwa is rich!" ending. The Steam guide also says you have to keep going on dates and kissing her until she says "that's all you get rid now!", so I did this to be safe
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13. Philia singing
14. Philia in her server outfit
15. Philia looking glamorous
16. Philia holding a baby
17. Philia with her back to the camera
18. Philia looking VERY provocative
Luckily all of these are story related for the Philia's After Story DLC. Just keep going through the stories and choose the Wedding ending when you have to make a decision at the end of the DLC.